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How to warm up inboxes after migrating to Inframail: why migrated inboxes need re-warming, the exact warmup timeline, what changes vs. a fresh inbox, and common mistakes.
Priya Nair
B2B growth marketer, ex-Apollo user · Updated June 24, 2026
Last updated: June 2026 · Priya Nair, B2B growth marketer, ex-Apollo user
TL;DR — 7 things to know before reading
Migration warmup is the most frequently skipped step in infrastructure transitions, and it is the step most likely to cause campaign disruption. The assumption practitioners make is logical but wrong: if an inbox was sending 50 cold emails per day with good deliverability at Provider A, it should be able to send 50 cold emails per day with good deliverability at Provider B immediately after migration. The assumption is wrong because deliverability is not just a property of the email address — it is a property of the combination of address, sending IP, DKIM signature, and the relationship between that combination and the reputation databases recipient providers maintain.
When you move to Inframail from a previous provider, the email address is the same but everything else changes. The sending IP addresses are different (Microsoft 365 pools rather than your previous provider's pools). The DKIM selector and key are different (Inframail generates new keys rather than porting your previous ones). The SMTP relay path is different. Recipient providers that had learned to trust your previous combination of address + IP + DKIM see a changed combination and must re-learn. The warmup period is the time it takes for this re-learning to happen.
The good news is that migrated inbox warmup is faster than new inbox warmup precisely because the email address carries reputation. A domain and address that have been sending legitimate email for 12 months has accumulated positive signals that partially transfer even when the infrastructure changes. The bad news is that teams that skip migration warmup entirely lose those accumulated signals quickly when the first campaign from the new infrastructure generates deliverability problems that would have been avoided.
Inframail handles the Microsoft 365 infrastructure post-migration. Instantly provides the warmup engine that rebuilds reputation signals on the new infrastructure. Quarvio provides the clean contact data that keeps bounce rates low during the critical early sends on the new infrastructure. Aimfox runs LinkedIn outreach in parallel during the migration warmup period when email volume is reduced.
Understanding why migration warmup is necessary prevents the temptation to skip it when timelines are tight.
Sending IP addresses change completely. Your previous provider had a specific pool of IP addresses from which your emails were relayed. Inframail's Microsoft 365 infrastructure uses Microsoft's Office 365 server IP ranges. These are different IP addresses with a different IP reputation profile. Recipient mail servers that maintained positive reputation data for your previous provider's IPs see new IPs and have no established positive reputation for them in the context of your domain.
DKIM keys and selectors change. DKIM authentication involves a public-private key pair. Your previous provider generated a key pair, published the public key to your domain's DNS (as a CNAME or TXT record), and signed outgoing emails with the private key. Inframail generates a completely new key pair for Microsoft 365. Your DNS is updated with the new public key (Inframail's DKIM CNAME record). The old key pair is no longer used. Recipient mail servers that had seen many properly DKIM-signed emails from your address using the old key now see the new key signature for the first time. It passes authentication (the math checks out), but it lacks the history of verified sends that builds trust in reputation systems.
SPF include paths change. Your SPF record previously permitted your old provider's server IPs to send on behalf of your domain. Inframail's SPF record includes Microsoft 365's server ranges instead. The net effect is that your SPF record now authorises a different set of IPs. Recipient servers that were familiar with your old SPF authorisation see a new one.
DMARC alignment must be verified for the new setup. DMARC passes when SPF or DKIM aligns with the From header domain. After migration, this alignment should still be present, but it must be verified because the new DKIM selector must align with the From domain for DMARC to pass on DKIM alignment.
The email address and domain reputation. The From address (name@yourdomain.com) is the same. The sending domain (yourdomain.com) is the same. Google and Microsoft's reputation systems attribute positive sending history to both the domain and the email address, and this history persists across infrastructure changes. A well-warmed domain with 6 months of positive sending history at Provider A carries that reputation advantage to Provider B.
Your list and contact relationships. Prior recipients who previously received and opened your emails are more likely to open future emails from the same address, even from a new infrastructure. Their positive engagement history with your address is stored in their email client's contact data.
The net effect: migrated inboxes start the re-warming period with partial credit from their prior reputation. A completely new inbox starts from zero. This is why migration warmup takes 7–14 days rather than 28–60 days.
Inframail setup is complete and verified. DNS records (SPF, DKIM, DMARC) must be passing before warmup begins. Running warmup on an inbox with broken authentication produces no useful reputation signals — the emails fail authentication checks and the warmup sends contribute no positive reputation value.
The old sending infrastructure is still running in parallel (temporarily). Do not disconnect from your previous provider until the Inframail setup is confirmed working and at least one test send has gone through successfully. The transition period should overlap: old provider continues while Inframail is being set up, then traffic migrates after Inframail is confirmed.
Warmup tool is configured. Inframail does not have a built-in warmup engine. You need Instantly or another warmup tool configured and connected to the migrated Inframail inbox before beginning. Instantly's warmup pool is large and well-managed; it is the recommended option for this stack.
Campaign sending is paused from the migrated inbox. During the 7–14 day migration warmup period, the migrated inbox should send warmup emails only — no cold campaigns. This is the most important operational rule and the one most frequently ignored. Running campaigns during migration warmup compounds the new-IP trust-building challenge by adding cold outreach rejection signals before positive reputation is re-established.
Complete the Inframail setup for the migrated inbox: connect the domain, verify DNS records, create the inbox with the same username as the migrated address, export SMTP/IMAP credentials, and connect to Instantly.
Verify authentication passes from the new infrastructure: send a test email to a personal Gmail or Outlook account and check the email headers. SPF, DKIM, and DMARC should all show as "pass." If any authentication header shows "fail" or "none," resolve before proceeding.
Check the MXToolbox blacklist status for the sending domain and for the Microsoft 365 IP ranges used. While Microsoft 365 IPs generally have clean reputation, verify before investing in warmup.
Enable Instantly's warmup for the migrated inbox. Set initial warmup volume to 20–30 emails per day (higher than new inbox warmup because the address has prior reputation that provides some initial trust).
Send zero cold campaign emails from the migrated inbox during this phase. All sends should come from the warmup engine only.
At the end of day 7, check: (1) warmup score in Instantly — should be at or above 80 on Instantly's scoring scale; (2) inbox placement test from the migrated inbox — should show inbox at Gmail and Outlook; (3) authentication headers on a test send — all three should show as pass.
If these checks pass, proceed to Phase 3. If warmup score is below 70 or inbox placement is showing significant spam (above 30%), extend Phase 2 by another 7 days.
Begin cold campaign sends from the migrated inbox at 10–15 emails per day. Keep warmup running simultaneously at its current volume (20–30 warmup emails per day).
The warmup emails continue to generate positive engagement signals (opens, replies, non-spam marks) that partially offset the cold outreach signals.
Monitor daily: bounce rate (target below 3% in this phase), complaint rate (target below 0.1%), open rate (compare to the inbox's pre-migration performance as a baseline). If any metric exceeds these thresholds, reduce campaign volume or pause and investigate.
At the end of day 14, if metrics are healthy, the inbox is ready for normal campaign volume.
Gradually increase campaign sends to full pre-migration volume over 5–7 additional days. Avoid jumping from 15 emails per day to 50 emails per day in one step; increase by 5–10 emails per day.
Continue warmup indefinitely alongside campaigns. Warmup is not a one-time event; it provides ongoing positive engagement signal that should run for the lifetime of the inbox.
What to do: Set up the migrated inbox in Inframail exactly as you would for a new inbox, but use the same email address (username) as the address being migrated.
Sub-steps:
Benchmark: Migration setup for a single inbox that has an already-verified domain takes 15–20 minutes.
Failure mode: Creating the inbox with a different username than the migrated address. If you were previously sending from ryan@yourdomain.com and create ryan2@yourdomain.com in Inframail, the prior domain reputation partially transfers but the address-level reputation does not.
What to do: Add the migrated inbox to Instantly using SMTP/IMAP credentials and enable warmup.
Sub-steps:
Benchmark: Connection and warmup activation takes 5–10 minutes per inbox.
Failure mode: Using the warmup volume settings recommended for a completely new inbox (10–15 per day). Migrated inboxes have prior reputation and can start warmup at higher volume. Starting too low extends the warmup period unnecessarily.
What to do: Keep campaign sending paused from the migrated inbox for 7 days while warmup runs.
Sub-steps:
Benchmark: Warmup score above 70 on Instantly by day 7 indicates the new infrastructure is establishing positive reputation. Score below 60 on day 7 indicates an authentication issue or other problem requiring investigation.
Failure mode: Running campaign emails from the migrated inbox during the warmup-only phase. Even a small number of campaign sends during this period adds rejection and negative engagement signals that counteract warmup progress. Strictly enforce the warmup-only rule for the first 7 days.
What to do: After 7 days of successful warmup-only operation, begin reintroducing cold campaign sends at reduced volume while warmup continues.
Sub-steps:
Benchmark: Healthy metrics during this phase: bounce rate below 3%, complaint rate below 0.1%, open rate within 5 percentage points of pre-migration baseline.
Failure mode: Starting at full pre-migration campaign volume (e.g., jumping to 50 cold emails per day from day 8). The new sending infrastructure has not yet accumulated sufficient positive reputation to support that volume without deliverability risk.
What to do: Gradually increase campaign sends from 10–15 per day to full pre-migration volume over the final phase.
Sub-steps:
Benchmark: By day 21, a successfully migrated inbox should be at or near full pre-migration campaign volume with metrics at or above pre-migration levels.
Failure mode: Treating day 21 as the finish line and turning off warmup. Warmup should continue at reduced volume (10–20 warmup emails per day) indefinitely alongside campaigns.
| Phase | Duration | Daily warmup sends | Daily campaign sends | Key check |
|---|---|---|---|---|
| Infrastructure setup | Day 0 | 0 | 0 | DNS all pass, test send confirmed |
| Warmup-only | Days 1–7 | 20–30 | 0 | Warmup score 70+ by day 7 |
| Gradual reintroduction | Days 8–14 | 20–30 | 10–15 | Bounce <3%, complaint <0.1% |
| Scale phase | Days 15–21 | 15–20 | Increase to full volume | Open rate within 5pts of baseline |
| Steady state | Day 22+ | 10–20 ongoing | Full pre-migration volume | Weekly metrics review |
If you are migrating multiple inboxes from a previous provider to Inframail, migrate in batches of 3–5 inboxes per week rather than migrating all at once. This approach:
Keeps some inboxes running campaigns at full volume during the migration period (the un-migrated inboxes continue operating normally), so total campaign volume is not interrupted. Limits the number of inboxes in warmup-only mode simultaneously, reducing the campaign volume reduction. Provides learning from the first batch before migrating subsequent batches — if the first batch of 3 inboxes encounters unexpected issues during migration warmup, you can adjust the protocol before migrating the remaining 20 inboxes.
For each batch, complete the full warmup protocol before migrating the next batch. Staggering by one week per batch means a 20-inbox migration takes 4–7 weeks total but maintains continuous campaign volume throughout.
The 7-day warmup-only phase is the ideal time to audit and clean the contact list you will use with the migrated inbox. During warmup, no campaign sends are going out, so there is no risk of a bad list damaging the inbox during its most vulnerable period.
Run your contact list through a verification tool (or re-order verified contacts from Quarvio) during the warmup phase. By the time Phase 3 begins and campaign sends start, you have a freshly verified list that will produce low bounce rates during the critical first campaign sends on the new infrastructure.
The combination of a properly warmed migrated inbox and a freshly verified contact list is the configuration most likely to produce deliverability metrics at or above pre-migration levels from the first campaign day.
Before migrating to Inframail, update your DMARC policy to p=quarantine or p=reject if it is currently at p=none. A stronger DMARC policy tells recipient providers that you actively manage authentication for your domain and that unauthenticated mail from your domain should be blocked. This policy signal improves your domain's treatment at recipient providers independent of the infrastructure migration.
After migration, the new Inframail DKIM signature will pass DMARC under the new infrastructure's key. Recipient providers see both the DMARC policy signal and the properly authenticated new signature, which accelerates trust-building during the migration warmup period.
The first 7 days after migration are when blacklist risk is highest: the new sending IPs have no established reputation, and if something goes wrong (bad list, complaint spike), they can be blacklisted quickly. During the warmup-only phase, check MXToolbox blacklist checker for the sending domain daily.
If a blacklist appearance occurs during warmup-only sends, it indicates a problem with the warmup pool itself or an authentication issue — not campaign complaints, since no campaigns are running. Report the blacklist appearance to Inframail support and pause warmup until the issue is investigated.
Before migrating, export campaign performance data for the migrated inbox: open rate, reply rate, bounce rate, and inbox placement rate. This data becomes the baseline you compare against post-migration to determine whether the migration warmup was successful.
A successful migration warmup should produce post-migration metrics within 10 percentage points of pre-migration metrics by day 21. If post-migration metrics are significantly worse (open rate drops more than 15 points, bounce rate increases more than 2%), the migration warmup has not been completed successfully and additional warmup time or investigation is needed.
Symptoms: After 7 days of warmup-only sends, the Instantly warmup score is below 60 or has been flat for 3+ days.
Diagnosis steps:
Fix: Fix IMAP connectivity first if that is the issue. Verify DKIM authentication from an email header check. Increase daily warmup volume to 30–40 per day if it has been set too low.
Symptoms: Inbox placement tests showed 80%+ inbox rate during warmup-only phase but dropped to 40–50% inbox rate after cold campaign sends began.
Diagnosis steps:
Fix: The most likely cause is contact list quality. Pause campaigns immediately. Verify the contact list quality before resuming. If contacts are verified and the issue persists, reduce campaign volume and extend the gradual reintroduction phase.
Symptoms: Pre-migration test emails showed DKIM signed with "selector1._domainkey" but post-migration test emails show a different selector or DKIM fail.
Diagnosis steps:
Fix: Add the correct DKIM CNAME record from Inframail to your DNS panel and wait for propagation. Do not remove old records before the new ones are confirmed as propagated.
Symptoms: During the gradual reintroduction phase, complaint rates are above 0.3% even at low campaign volume (10–15 emails per day).
Diagnosis steps:
Fix: Ensure the From name and address matches exactly what recipients previously received from this inbox. Start the campaign reintroduction with your most engaged, highest-quality contact segments rather than lower-engagement or unverified segments.
Symptoms: During migration, the old provider's platform shows errors or blocks sends from the domain while Inframail is being set up.
Diagnosis steps:
Fix: During migration transition, maintain both the old provider's SPF include and Inframail's SPF include temporarily (both can coexist in an SPF record by adding both include statements). Similarly, keep the old DKIM record active until you are certain no emails are routing through the old provider. Remove the old records only after migration is complete.
Symptoms: After completing migration warmup, the migrated inbox has lower open rates and worse inbox placement than brand-new inboxes on the same Inframail domain.
Diagnosis steps:
Fix: If the migrated address had poor pre-migration reputation, migration warmup will not overcome that negative history. In this case, the inbox needs an extended warmup period (28–60 days rather than 7–14 days) to rebuild positive reputation signals, or it should be retired and replaced with a new inbox address that starts from a neutral reputation baseline.
A verified user on Inframail reviews on G2:
"I migrated 15 inboxes from a competitor to Inframail and skipped the re-warming step because I thought the inboxes were already warm. Within two weeks, open rates dropped from 28% to 14% and I had three inboxes flagged for spam. Re-ran warmup for 14 days on all inboxes and recovered. The migration warmup is not optional."
— Verified buyer on Inframail reviews on G2
From a thread in r/coldemail (778 upvotes):
"Moved from Google Workspace to Inframail for the cost savings. The inboxes had been sending cold email for 8 months at the old provider with good deliverability. At Inframail I ran a 10-day warmup before launching campaigns and it was completely smooth. Deliverability was actually slightly better post-migration. The warmup step is what makes the difference."
— r/coldemail, 778 upvotes
A second reviewer on Instantly reviews on G2, where Instantly holds 4.9/5 from 2,800+ reviews:
"Instantly's warmup network is what I use for all migrations, not just new inbox setup. The warmup score gives you a clear signal of when a migrated inbox is ready to handle campaign volume. I will not add an inbox to a campaign until its warmup score is above 75 in Instantly. That threshold has worked consistently across 30+ migrations."
— Verified buyer on Instantly reviews on G2
| Need | Tool | Notes |
|---|---|---|
| Migrated inbox infrastructure | Inframail | Microsoft 365, auto DNS, SMTP credentials |
| Warmup management | Instantly | Warmup score tracking, large pool |
| Campaign sending | Instantly | Post-migration campaigns after warmup completes |
| Verified contact data | Quarvio | Clean lists during critical first sends |
| LinkedIn outreach | Aimfox | Maintain LinkedIn volume during email migration |
Do I really need to re-warm inboxes after migrating to Inframail if they were already warm at the previous provider?
Yes. The email address retains reputation, but the sending IP, DKIM signature, and SPF include path all change when you migrate to Inframail. Recipient mail servers that previously trusted your specific combination of address + IP + DKIM see a new combination and must re-learn it. The re-warming period for migrated inboxes (7–14 days) is shorter than for new inboxes (28–60 days) because the address reputation carries over, but it cannot be skipped entirely.
How long does migration warmup take compared to new inbox warmup?
Migrated inbox warmup: 7–14 days before moderate campaign volume, 21 days to full campaign volume. New inbox warmup: 14–28 days before moderate campaign volume, 30–60 days to full campaign volume. The difference is that migrated addresses have existing positive reputation signals that accelerate the re-trust process.
Can I run campaigns on un-migrated inboxes while warming up the migrated ones?
Yes. This is the recommended migration strategy: keep un-migrated inboxes running campaigns at full volume while the migrated inboxes complete warmup. Migrating in batches of 3–5 at a time maintains continuous campaign volume with minimal disruption.
What tool should I use for warmup during migration to Inframail?
Instantly is the recommended warmup tool for migrated Inframail inboxes. Connect the migrated inbox to Instantly via SMTP/IMAP and enable Instantly's warmup feature. Instantly's warmup network is large, the warmup score is a reliable metric for assessing warmup progress, and the same tool can be used for campaigns after warmup is complete.
What is the minimum warmup time before campaigns can resume on a migrated inbox?
7 days of warmup-only sends, followed by gradual campaign reintroduction at 10–15 emails per day. The warmup score in Instantly should be above 70 and inbox placement tests should show 70%+ inbox rate before any campaign sends begin. If either of these benchmarks is not met at day 7, extend the warmup-only phase by another 7 days.
Should I migrate all inboxes simultaneously or in batches?
Batches of 3–5 inboxes per week is the recommended approach. Migrating all inboxes simultaneously reduces total campaign capacity to zero during the warmup period. Batch migration keeps un-migrated inboxes running at full volume while migrated inboxes complete warmup, maintaining campaign continuity.
What happens to my email address reputation when I migrate to Inframail?
The email address and domain reputation carry over partially. Reputation signals that are tied to the address itself (positive engagement history from previous sends) persist. Reputation signals tied to the IP and infrastructure (IP reputation, specific DKIM history) do not carry over and must be rebuilt through warmup on the new infrastructure. This is why migrated inboxes start warmup at a higher volume (20–30 emails per day) than new inboxes (10–20) — they have a head start from address-level reputation.
How do I know when migration warmup is complete?
Three signals confirm migration warmup is complete: (1) warmup score in Instantly is above 75, (2) inbox placement test shows 70%+ inbox rate at Gmail and Outlook, and (3) per-inbox metrics during the gradual campaign reintroduction phase (days 8–14) show bounce rate below 3% and complaint rate below 0.1%. All three must be met before scaling to full campaign volume.
Can I keep my old email address exactly when migrating to Inframail?
Yes. Inframail creates inboxes based on the username and domain you specify. If you were previously sending from sarah@yourdomain.com, create "sarah" as the inbox username on yourdomain.com in Inframail. The From address will be identical to the pre-migration address.
What contact list should I use for the first campaign sends after migration?
Use your most engaged and cleanest contact segments for the first post-migration campaign sends (days 8–14). This provides the best possible engagement signals and lowest bounce risk during the critical period when the new infrastructure is establishing reputation. Reserve lower-quality or less-engaged segments for later when the inbox reputation is fully re-established. Pre-verified contacts from Quarvio are the safest option for these first sends.
Keep your contact list clean during migration
The most common cause of failed migration warmup is launching campaigns against unverified contacts during the vulnerable re-warming period. Verified contact data is essential for the first sends on the new infrastructure.
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