How to create unlimited inboxes with Inframail: inbox scaling architecture and management

Last updated: June 2026 · Sarah Okonkwo, Cold email strategist, 300+ domain setups

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TL;DR — 7 things to know before reading

• Inframail's flat per-domain pricing means you can create as many inboxes as needed on a given domain at the same monthly cost; the economics improve dramatically as you scale past 5–10 inboxes
• The practical limit per domain is not Inframail-imposed but deliverability-driven: more inboxes on one domain increases the reputation concentration risk; the recommended range is 3–5 inboxes per domain for balanced risk and cost
• The domain math for scaling: at 40 emails per inbox per day, 5 inboxes per domain, you need 1 domain per 200 daily emails; for 1,000 emails per day you need 5 domains with 25 total inboxes
• Inframail's bulk SMTP export generates a CSV of all inbox credentials, which you then upload to Instantly or Smartlead in bulk rather than connecting inboxes one by one
• Warmup capacity is the bottleneck for scaling, not inbox creation: you can create 50 inboxes in an hour, but each inbox needs 14–60 days of warmup before campaigns; plan your inbox creation timeline backwards from your campaign launch date
• Domain age matters at scale: registering all domains on the same day and immediately launching campaigns creates a cluster of zero-age domains that perform worse than a portfolio built up over time; stagger domain registrations
• Verdict: build your inbox portfolio on Inframail, manage campaigns across the portfolio in Instantly, use Quarvio for contact data scaled to match your inbox capacity, and Aimfox for LinkedIn alongside email

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Our take

The phrase "unlimited inboxes" is technically accurate for Inframail — the platform does not cap the number of inboxes you can create on a domain — but the practically useful question is not "how many can I create" but "how many should I create and on how many domains." Those are inbox architecture decisions, not product capability questions.

After 300+ domain setups, the inbox architecture mistakes that consistently produce deliverability problems are: too many inboxes on too few domains (concentrating reputation risk), too many inboxes created at once before warmup capacity is available to handle them (launching campaigns from half-warmed inboxes), and inbox sprawl without documentation (losing track of which inboxes are assigned to which campaigns, which are in warmup, and which have had problems). These mistakes are not inevitable; they result from treating inbox creation as a purely mechanical process rather than an architectural one.

This guide covers the architecture: how to calculate the right number of domains and inboxes for your volume target, how to sequence inbox creation against warmup timelines, how Inframail's bulk provisioning tools make the creation process efficient, and how to build a management system that keeps a large inbox portfolio organised. The mechanics of creating a single inbox take five minutes; building an inbox portfolio that can sustain 1,000+ daily emails for months without deliverability problems takes planning.

Inframail provides the Microsoft 365 inboxes and bulk provisioning tools. Instantly provides the campaign management, warmup, and bulk inbox connection tools. Quarvio delivers the contact data at the scale the inbox portfolio can handle. Aimfox adds LinkedIn outreach that runs in parallel.

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The domain math: how many inboxes and domains do you need?

Before creating any inboxes, calculate your requirements based on your daily send target.

The sending capacity formula

The core formula:

Inboxes needed = Daily send target ÷ emails per inbox per day
Domains needed = Inboxes needed ÷ inboxes per domain

The standard parameters:

• Emails per inbox per day: 40–50 for fully matured inboxes; 10–20 for inboxes in early campaign phase (weeks 5–8 post-warmup)
• Inboxes per domain: 3–5 (for reputation isolation; higher concentrates risk)

Example calculations for common volume targets:

Why inboxes per domain matters for reputation

The deliverability reason to limit inboxes per domain to 3–5 is reputation concentration risk. If a campaign sends from 20 inboxes on one domain and that campaign has a high complaint rate, the entire domain's reputation suffers. With 5 inboxes per domain, the worst case is one domain's inboxes being paused, while 4 other domains' inboxes continue operating normally.

The cost reason to potentially increase inboxes per domain is Inframail's flat pricing. At 10 inboxes per domain, the per-inbox cost is lower than at 5 inboxes per domain. For teams with strong list quality and campaign control (low complaint rates, verified contacts), 8–10 inboxes per domain may be acceptable. For teams with variable list quality, 3–5 per domain limits potential damage.

The warmup capacity constraint

The most common mistake in scaling inbox creation is not calculating warmup capacity. You can create 25 inboxes today, but you cannot start 25 warmup sequences simultaneously and have all 25 ready for campaigns in 14–28 days. Consider:

• Warmup requires active management: monitoring warmup scores, identifying inboxes with problems, adjusting volume
• Instantly can warm up many inboxes simultaneously, but each warmup score must be individually monitored
• Starting 25 simultaneous warmups means 25 inboxes in a vulnerable early-warmup state at the same time; if something goes wrong with the warmup pool or DNS, all 25 are affected

The practical approach: create and warm inboxes in batches of 5–10, wait for each batch to complete warmup and reach campaign-ready status before creating the next batch. This produces a slower scale-up but a more reliable portfolio.

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Step-by-step: creating and scaling inboxes in Inframail

Step 1: Calculate your inbox and domain requirements

What to do: Before creating any inboxes, calculate the total number of domains and inboxes required for your volume target.

Sub-steps:

1. Define your 90-day daily send volume target: how many cold emails per day do you plan to send by the end of 90 days?
2. Apply the formula: divide by 40 emails per inbox per day to get total inboxes needed
3. Divide inboxes by 4 (inboxes per domain) to get total domains needed
4. Apply the warmup timeline: if each batch of 5 inboxes takes 28 days to complete warmup, work backwards from your campaign launch date to determine when inbox creation batches must begin
5. Document the plan in a spreadsheet before purchasing domains or creating inboxes

Benchmark: A 1,000 email/day target at 90 days requires 25 inboxes across 6–7 domains, with the first batch of domains purchased and DNS configured today, and inbox creation happening in 5-inbox batches over 4–6 weeks.

Failure mode: Calculating inboxes without calculating domains, then creating all inboxes on one or two domains and concentrating deliverability risk unnecessarily.

Step 2: Purchase and configure sending domains

What to do: Purchase the required number of sending domains and configure their DNS for Inframail before creating any inboxes.

Sub-steps:

1. Purchase domains from your registrar (Namecheap, Cloudflare Registrar, or similar) using a consistent naming convention: teamacme.com, meetacme.com, getacme.com, joinacme.com, etc.
2. Add each domain to Inframail by navigating to Domains and selecting "Add Domain"
3. For each domain, copy the generated SPF, DKIM, and DMARC records to your registrar's DNS panel
4. Wait for DNS propagation (15–60 minutes per domain) and verify all records pass in Inframail before creating inboxes on that domain
5. Document each domain with its purchase date, registrar, and DNS verification status

Benchmark: Setting up 5 domains (purchasing, adding to Inframail, and adding DNS records) takes 2–3 hours including propagation wait time.

Failure mode: Creating inboxes on a domain before DNS is verified. Inboxes created on an unverified domain will not have proper DKIM signing, and warmup and campaign sends will fail authentication.

Step 3: Create inboxes in the first batch (5 per domain)

What to do: Create the first batch of inboxes on your first verified domain. Start with 4–5 inboxes on the first domain; do not create all inboxes on all domains simultaneously.

Sub-steps:

1. In Inframail, navigate to Inboxes and select "Create Inbox"
2. Select your first verified domain from the domain dropdown
3. Enter the inbox username; use professional human first-name patterns: sarah, ryan, james, marcus, priya, mike, alex, kate (avoid generic usernames like info, contact, hello, mail)
4. Create 4–5 inboxes on the first domain
5. Note the SMTP and IMAP credentials for each inbox from the Inframail dashboard; you will need these for the Instantly connection

Benchmark: Creating 5 inboxes on one domain takes 10–15 minutes.

Failure mode: Using generic usernames (info@, contact@, team@, sales@) which have higher complaint rates because they are commonly used for opt-in and marketing email and recipients frequently mark them as spam.

Step 4: Export SMTP credentials from Inframail

What to do: Use Inframail's bulk SMTP export to generate credentials for all created inboxes at once.

Sub-steps:

1. In Inframail, navigate to the SMTP settings or bulk export section
2. Generate the bulk SMTP export for all inboxes on your domain (or all domains if you have set up multiple)
3. Download the resulting CSV file; it contains one row per inbox with: email address, SMTP password, SMTP server, SMTP port, IMAP server, IMAP port
4. Save this file securely — it contains credentials for all your sending inboxes
5. Use this CSV as the reference document when connecting inboxes to Instantly

Benchmark: Bulk export for 25 inboxes takes 3–5 minutes and generates one CSV file.

Failure mode: Exporting individual inbox credentials one at a time when the bulk export is available. Manual credential copying for 25+ inboxes is time-consuming and error-prone.

Step 5: Connect inboxes to Instantly in bulk

What to do: Connect all Inframail inboxes to Instantly using the bulk SMTP export CSV rather than adding inboxes one at a time.

Sub-steps:

1. Log in to Instantly and navigate to Sending Accounts
2. Look for a bulk import option (Instantly's interface may call this "bulk add", "CSV import", or similar)
3. Upload the SMTP CSV exported from Inframail
4. Instantly will attempt to connect each inbox and report success or failure per row
5. For any failed connections, check the specific credential row in the CSV for typos and retry manually
6. After import, confirm that all connected inboxes show as "connected" rather than "failed" in the Sending Accounts view

Benchmark: Bulk import of 25 inboxes into Instantly takes 10–20 minutes including troubleshooting any failed connections.

Failure mode: Connecting inboxes one at a time when bulk import is available. At 5 minutes per inbox for 25 inboxes, manual connection takes over 2 hours; bulk import reduces this to 15 minutes.

Step 6: Enable warmup for all connected inboxes

What to do: Activate Instantly's warmup feature for all newly connected inboxes before running any campaign sends.

Sub-steps:

1. In Instantly, navigate to Sending Accounts and identify all inboxes showing as connected
2. Enable warmup for each inbox; Instantly may allow bulk warmup enabling for multiple inboxes simultaneously
3. Set the initial daily warmup volume: 15–20 emails per day for the first week, scaling to 30–40 per day by week 3
4. Confirm warmup is active and sending for each inbox by checking the warmup status dashboard
5. Do not assign any inbox to a campaign until its warmup has been running for at least 14 days

Benchmark: Enabling warmup for 25 inboxes takes 15–30 minutes depending on whether bulk warmup activation is available.

Failure mode: Not enabling warmup for some inboxes in the batch because the interface is slow or you are fatigued from the bulk setup process. Every inbox that misses warmup and gets added to campaigns prematurely risks reputation damage.

Step 7: Build the inbox management documentation

What to do: Create and maintain a documentation system for your inbox portfolio before inboxes become too numerous to manage by memory.

Sub-steps:

1. Create a spreadsheet with one row per inbox containing: email address, domain, username, date created, warmup start date, estimated campaign-ready date, current status (warmup / campaign / paused / retired), assigned campaign, warmup score (updated weekly), bounce rate (updated weekly), notes
2. Create a separate tab for domains: domain name, registrar, purchase date, DNS verified date, inboxes on this domain, notes
3. Update the spreadsheet weekly: warmup scores from Instantly, bounce rates from campaign analytics
4. Use the campaign-ready date column to know when each inbox graduates from warmup to campaign-eligible status

Benchmark: Spreadsheet setup takes 30–60 minutes. Weekly maintenance takes 15–30 minutes. Without documentation, managing more than 15 inboxes becomes operationally unmanageable.

Failure mode: Relying on memory for inbox status across a portfolio of 20+ inboxes. Without documentation, inboxes in different stages (warmup, campaign, paused, retired) become indistinguishable, and it becomes easy to accidentally add an unwarmed inbox to a campaign.

Step 8: Create subsequent inbox batches on a staggered schedule

What to do: Continue creating inboxes in batches as earlier batches complete warmup and become campaign-ready, rather than creating all inboxes at once.

Sub-steps:

1. After the first batch of 5–10 inboxes has been warming for 14 days, create the second batch on the second domain
2. Continue the pattern: each new batch starts warmup while the previous batch moves to campaigns
3. By week 8, with 5 batches of 5 inboxes each having staggered warmup starts, you have 25 inboxes across 5 domains with staggered campaign maturity
4. Document each new batch in your inbox management spreadsheet
5. Register new domains 30–60 days before they need to be used, to allow domain aging

Benchmark: A staggered schedule producing 25 campaign-ready inboxes takes 8–10 weeks from the first domain registration. This is significantly longer than creating all 25 inboxes today, but produces a more reliable, better-aged portfolio.

Failure mode: Creating all 25 inboxes on day 1 and trying to warm them all simultaneously. This creates 25 simultaneous warmup sequences, all of which complete simultaneously 4 weeks later, followed by 25 inboxes needing campaigns at the same time — an all-or-nothing approach that lacks the risk distribution of staggered creation.

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Inbox scaling configuration reference

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Advanced tactics

Tactic 1: Build a domain aging pipeline

Register new domains continuously, 30–60 days before you need them. Even at the minimum warmup volume (10–15 emails per day), a domain that has been sending for 60 days before campaign launch has materially better inbox placement than a day-zero domain.

The practice: every time you launch a new batch of inboxes, also purchase the next batch of domains (to be used 60 days from now) and start them on minimal warmup. By the time you need them, they will be aged and partially warmed. The pipeline runs continuously: create, age, warm, campaign, repeat.

This requires forward planning and a disciplined domain registration schedule, but the payoff is a portfolio where no domain is ever launched cold from day zero.

Tactic 2: Segment domains by campaign type, not just by volume

Rather than assigning inboxes to campaigns randomly, segment domains by campaign type. One domain for first-touch campaigns (highest risk, newest contacts), a separate domain for re-engagement campaigns (lower risk, previously contacted), another for warm referral outreach (lowest risk, highest quality contacts).

First-touch campaigns to unverified or marginally verified contacts carry the highest complaint and bounce risk. Concentrating this risk on dedicated domains protects the domains used for higher-quality sends. If the first-touch domain accumulates complaints, it can be paused without affecting the warmer outreach domains.

Tactic 3: Retire and replace inboxes on a 12-month cycle

No inbox lasts forever. Inboxes that have been sending cold email for 12+ months accumulate complaint history, engagement fatigue signals, and potentially appear on recipient blocklists as recognised cold email senders. Proactively retiring inboxes at the 12-month mark and replacing them with fresh inboxes on the same domain maintains portfolio freshness.

The operational process: when an inbox reaches 12 months, add it to a "retirement queue", reduce its campaign volume to zero over 2 weeks, then deactivate it in Inframail. Simultaneously create a new inbox on the same domain (or a domain added 60 days ago) and begin warmup. This maintains constant inbox count while cycling out aged inboxes for fresh ones.

Tactic 4: Monitor cost-per-contact at scale and adjust domain/inbox count accordingly

At scale, the inbox cost is the dominant operational cost in your cold email infrastructure (ahead of the campaign tool subscription and contact data cost). Calculate cost per contact at each volume tier:

Inframail cost per month ÷ total daily sends × 30 ÷ 1 = cost per email sent. Compare this to the cost per reply or cost per meeting booked. If the inbox infrastructure cost is a meaningful percentage of revenue per meeting booked, the incentive to optimise inbox count and per-inbox send volume is clear.

As you scale, optimise by: increasing sends per inbox toward the safe maximum (40–50), increasing inboxes per domain if list quality is strong (reducing domain cost per inbox), and retiring underperforming inboxes rather than leaving idle inboxes in the portfolio.

Tactic 5: Create spare inboxes for emergency rotation

Maintain a 10–15% reserve of inboxes in warmup at all times, beyond your current campaign needs. These spare inboxes serve as emergency replacements when an active inbox has a deliverability problem and must be pulled from campaigns.

Without spare inboxes, pulling one problem inbox from a campaign reduces sending capacity by the full per-inbox daily volume. With spare inboxes in warmup, a replacement is available within days rather than weeks. The maintenance cost (warmup emails from spare inboxes) is low relative to the capacity assurance benefit.

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Troubleshooting inbox scaling issues

Problem 1: Some inboxes failing to connect to Instantly after bulk import

Symptoms: After bulk importing the Inframail SMTP CSV into Instantly, some inboxes show as "failed" or "connection error."

Diagnosis steps:

1. Check the specific credentials for the failed inboxes in the CSV — look for special characters in the password that may not have exported correctly
2. Verify the inbox exists in Inframail and is in "active" status
3. Try connecting the failed inboxes manually (not from the CSV) to identify whether the issue is the credential itself or the import format

Fix: Re-export credentials from Inframail for the failed inboxes and connect them manually. If manual connection also fails, the inbox may not have been fully provisioned in Inframail; contact Inframail support.

Problem 2: Warmup scores not improving across the new batch

Symptoms: After 14 days, warmup scores for a new batch of inboxes are below 60 and not improving.

Diagnosis steps:

1. Verify IMAP is correctly configured for each inbox (required for warmup reply detection)
2. Check whether DNS records passed verification for these inboxes' domain before warmup began
3. Check the Instantly warmup dashboard for any errors or pool issues affecting this batch

Fix: Fix IMAP connectivity for any inbox where IMAP shows as failed. If DNS is the issue, fix authentication records and restart warmup after records are confirmed passing.

Problem 3: Inframail not allowing more inboxes on a domain

Symptoms: When attempting to create inboxes beyond a certain count on a domain, Inframail shows an error or restricts creation.

Diagnosis steps:

1. Check your Inframail plan: some plan tiers have inbox count limits per domain
2. Check whether the issue is a temporary Microsoft 365 provisioning delay rather than a hard limit
3. Review Inframail's current plan documentation for per-domain inbox limits

Fix: Upgrade your Inframail plan if the current tier has inbox count limits that you have reached. Alternatively, distribute additional inboxes across more domains rather than adding more to a single domain.

Problem 4: All inboxes on one domain simultaneously showing deliverability problems

Symptoms: Multiple inboxes on the same domain all develop inbox placement problems at the same time.

Diagnosis steps:

1. Check the domain's overall reputation in Google Postmaster Tools
2. Check whether all inboxes are assigned to the same campaign that has a high complaint or bounce rate
3. Check the domain's MXToolbox blacklist status

Fix: If the entire domain is showing reputation problems, pause all campaigns from that domain. Investigate and fix the root cause (typically high complaint rate from a specific campaign or contact segment). The domain needs a 2–4 week sending pause combined with intensive warmup before resuming.

Problem 5: Running out of human-sounding inbox usernames

Symptoms: After creating many inboxes on the same domain, you have exhausted the list of professional first names and are resorting to less professional names.

Diagnosis steps:

1. Count the inboxes currently on the domain and compare to the 3–5 per domain recommendation
2. Check whether the domain has too many inboxes (above 10 per domain suggests this issue)

Fix: Add more domains rather than adding more inboxes to existing domains. The inbox username is a deliverability signal — professional first names perform better than generic or numbered names. If you need more inboxes, register new domains and use fresh first names on each domain.

Problem 6: Inbox management spreadsheet becoming unmanageable

Symptoms: The inbox portfolio spreadsheet has 100+ rows and tracking warmup status, bounce rates, and campaign assignments manually is taking significant time.

Diagnosis steps:

1. Assess whether the current spreadsheet is sufficient or whether you need a proper database solution
2. Check whether Instantly's analytics dashboard can replace some manual tracking

Fix: At 50+ inboxes, consider using Instantly's analytics as the primary monitoring tool and simplifying the spreadsheet to track only domain-level data and high-level inbox status (warmup / active / paused / retired). Reserve the detailed per-inbox tracking for problem inboxes only.

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What practitioners say about Inframail inbox scaling

A verified user on Inframail reviews on G2:

"Scaling from 10 to 50 inboxes was the point where Inframail's flat pricing made an obvious difference. At 50 inboxes with per-seat pricing at other providers, I would have been paying 5–10x more per month. The bulk SMTP export made connecting all 50 inboxes to Instantly manageable rather than a full-day manual task."

— Verified buyer on Inframail reviews on G2

A thread in r/coldemail (1,204 upvotes):

"The lesson I wish someone had told me at 50 inboxes: the bottleneck is never inbox creation, it's warmup management. Creating 50 new inboxes takes a day. Getting 50 inboxes to campaign-ready status takes 2 months with proper warmup. Plan your scale-up based on your warmup capacity, not your inbox creation speed."

— r/coldemail, 1,204 upvotes

A verified reviewer on Instantly reviews on G2, where Instantly holds 4.9/5 from 2,800+ verified reviews:

"We run 80 inboxes across 18 domains all managed through Instantly with Inframail for provisioning. The inbox documentation spreadsheet is the operational backbone — warmup scores, domain assignment, campaign assignment, problem history. Without that spreadsheet, 80 inboxes is unmanageable. With it, one person can handle the whole portfolio."

— Verified buyer on Instantly reviews on G2

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Our actual stack

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Frequently asked questions

How many inboxes can I create on one domain in Inframail?

Inframail does not impose a hard technical limit on inboxes per domain (the "unlimited" in the product name refers to this). However, the deliverability recommendation is 3–5 inboxes per domain to limit reputation concentration risk. Advanced users with strong list quality controls can run 8–10 inboxes per domain. Above 10 inboxes per domain, the risk of one campaign's problems affecting all inboxes on that domain becomes significant.

How many emails can I send per day per inbox?

40–50 emails per inbox per day is the standard limit for fully matured inboxes (8+ weeks of warmup). Per Woodpecker's guide on daily sending limits, 30–50 per day is the recommended range for cold email inboxes on properly warmed accounts. New inboxes in early campaign phase (weeks 5–8) should be capped at 10–20 per day.

How many inboxes do I need to send 1,000 emails per day?

At 40 emails per inbox per day, 1,000 daily emails requires 25 inboxes. At 50 per inbox per day (maximum for mature inboxes), 1,000 daily emails requires 20 inboxes. Plan for 25 inboxes across 6–7 domains to have margin for inbox maintenance, rotation, and some in permanent warmup mode.

How long does it take to build an inbox portfolio for 1,000 emails per day?

Approximately 8–12 weeks from the first domain registration to full volume. The timeline is determined by warmup requirements: each 5-inbox batch needs 28–60 days of warmup. If you create batches every 2 weeks, you reach 25 inboxes (5 batches × 5 inboxes) at week 10, with all 25 campaign-ready by week 10–14.

What should inbox usernames look like?

Professional first names: sarah, ryan, james, marcus, priya, mike, alex, kate, tom, emma. Avoid generic names (info, contact, hello, mail, team, sales) which carry higher complaint rates because they are recognisably generic. Avoid numbered variants (ryan1, ryan2) which look automated. On each domain, 4–5 different first names is sufficient.

Does Inframail support bulk inbox creation?

Inframail's interface allows creating inboxes one at a time, though this is fast (1–3 minutes per inbox). The bulk efficiency is on the export side: Inframail's bulk SMTP export generates credentials for all inboxes at once, which you then import into Instantly in bulk rather than connecting each inbox manually.

Can I use the same Inframail inboxes for both warmup and campaigns?

Yes. The same inbox runs warmup through Instantly's warmup engine while simultaneously sending campaign emails. Warmup should continue running throughout the inbox's active campaign period, not just during the initial warmup phase. The warmup emails generate positive engagement signals that partially offset cold outreach signals and help maintain reputation over time.

What happens if I exceed the recommended inboxes per domain?

There is no hard failure — the inboxes will function. The risk is reputation concentration: if one campaign from this domain generates high complaint rates or bounces, all inboxes on the domain share the reputation damage. With 10+ inboxes on one domain sending at 40+ emails per day each, one problematic campaign can generate 400+ complaints from a single domain, which degrades the domain's reputation for all inboxes and may trigger blocklisting.

How do I manage a large inbox portfolio without losing track of status?

A tracking spreadsheet with one row per inbox is the essential tool. The minimum columns: email address, domain, creation date, warmup start date, campaign-ready date, current status, assigned campaign, weekly warmup score, weekly bounce rate. Update weekly. At 50+ inboxes, consider using Instantly's analytics as the primary tracking tool and the spreadsheet for strategic overview.

What contact volume do I need to match a 25-inbox portfolio?

At 25 inboxes sending 40 emails per day, your capacity is 1,000 emails per day × 30 days = 30,000 emails per month. With a 3-step sequence per contact, you can reach 10,000 new contacts per month. Quarvio delivers 10,000 contacts at $199 — one order per month matches the capacity of a 25-inbox portfolio.

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Scale your inbox portfolio with verified contacts

Inbox capacity is only valuable when matched with sufficient clean contact data. At 25 inboxes sending 40 emails per day, you need approximately 10,000 fresh, verified contacts per month to maintain full sending capacity without re-contacting the same prospects too frequently.

Quarvio delivers pre-verified B2B contacts at $129 for 5,000 contacts through $699 for 50,000 contacts. 90% deliverability guarantee, 12-month credit validity, unused credits carry forward.

Start your contact order at Quarvio →