What is SMTP in cold email: how sending infrastructure actually works

Last updated: August 2026 · Priya Nair, B2B growth marketer, ex-Apollo user

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TL;DR — 5 things to know before reading

• SMTP (Simple Mail Transfer Protocol) is the technical protocol that carries email between servers — every email sent anywhere uses SMTP; it is the delivery mechanism, not a product or a service
• For cold email senders, the meaningful choice is not "which SMTP protocol" (it is universal) but "which infrastructure provider" handles the SMTP connection on your behalf — and that choice determines IP reputation, inbox rate, and operational complexity
• Bulk SMTP services (Mailgun, SendGrid) are designed for high-volume transactional email and require significant technical configuration for cold email use; they are not the recommended approach for B2B cold outreach
• Inframail provisions Microsoft 365 inboxes where the SMTP layer is handled by Microsoft, which carries established IP reputation with Gmail, Outlook, and other major mailbox providers — the technical SMTP configuration is invisible to the sender
• Instantly connects to any SMTP-compatible inbox via standard IMAP/SMTP credentials and manages sending sequences, warmup, and rotation across all connected inboxes regardless of where those inboxes are hosted

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Our take

SMTP appears in cold email discussions in ways that range from technically accurate to misleading. "SMTP server" sometimes means a raw bulk mail server, sometimes an inbox provider's sending infrastructure, and sometimes a feature in a sending tool. The confusion matters because these are different things with very different implications for cold email deliverability.

The protocol itself — SMTP — is not a product choice. It is the universal standard that all email uses. The relevant choice for cold email is what infrastructure sits behind the SMTP connection: which IP addresses are sending the email, what reputation those IPs carry, and what inbox and domain reputation has been built through warmup and authentication.

This guide explains what SMTP is in simple terms, how it fits into cold email infrastructure, and why the practical choice for most B2B cold email senders is dedicated inbox provisioning rather than direct SMTP service access.

What SMTP is (and is not)

SMTP stands for Simple Mail Transfer Protocol. It is the standard communication protocol for sending email between mail servers. Every email — from a transactional receipt to a marketing newsletter to a cold outreach email — is transmitted using SMTP.

When you send an email from any inbox (Gmail, Outlook, or a custom domain inbox), your email client connects to your mail server using SMTP and hands off the email. The mail server then uses SMTP to deliver the email to the recipient's mail server, which delivers it to the recipient's inbox.

SMTP is infrastructure-layer technology: it is a standard, not a product. There is no choice between "using SMTP" and "not using SMTP" for cold email — all email uses SMTP. The relevant choice is which service provides and manages the SMTP infrastructure.

What SMTP is not:

• Not a specific product or tool
• Not a deliverability advantage or disadvantage in itself
• Not something cold email senders configure directly in most modern setups
• Not the same as a bulk email service (though bulk email services use SMTP)

How SMTP fits into cold email infrastructure

A cold email infrastructure stack has four components (covered in detail in the cold email infrastructure guide):

1. Sending domain (domain name from which email is sent)
2. Email inboxes (accounts on those domains)
3. Authentication records (SPF, DKIM, DMARC)
4. Sending platform (Instantly)

SMTP is the protocol that the sending platform uses to connect to the inboxes and send email. When Instantly connects to a Microsoft 365 inbox provisioned via Inframail, it connects via IMAP (to read email, monitor replies, detect bounces) and SMTP (to send email sequences). The sender does not configure the SMTP protocol directly — Instantly and Inframail handle the connection.

The inbox provider — not the SMTP protocol itself — is what determines deliverability. Microsoft 365 inboxes send from Microsoft's IP infrastructure, which carries established reputation with all major mailbox providers. A generic bulk SMTP service sends from its own IP pool, which may or may not carry equivalent reputation, requires its own warmup, and requires the sender to manage IP reputation directly.

The three SMTP infrastructure options for cold email

Option 1: Dedicated inbox provisioning (recommended)

Provision individual email inboxes on a custom sending domain through a provider like Inframail. Each inbox is a full email account (username@sendingdomain.com) with its own SMTP and IMAP access, sending reputation, and warmup history.

The inbox provider's SMTP infrastructure is used to send email. Authentication (SPF, DKIM, DMARC) is configured per domain. The sending platform (Instantly) connects to each inbox via standard SMTP/IMAP and handles sequence sending, inbox rotation, and warmup.

Why this is the right approach for B2B cold email:

• Each inbox builds its own reputation independently through warmup
• Microsoft 365's IP infrastructure (used by Inframail) carries strong baseline trust with Gmail
• Inbox rotation across multiple warmed inboxes distributes reputation risk
• The sender controls warmup, sending limits, and domain lifecycle
• The SMTP connection is handled transparently by Inframail and Instantly

Option 2: Bulk SMTP services (not recommended for cold email)

Bulk SMTP services like Mailgun and SendGrid are designed for high-volume transactional email: receipts, notifications, password resets. They provide SMTP relay service where the sender's emails are delivered through the service's IP infrastructure.

These services are powerful tools for the use case they are designed for. They are not well-suited to B2B cold email because:

Volume thresholds: Mailgun and SendGrid impose per-day sending limits tied to the pricing plan. Getting above those limits requires plan upgrades proportional to volume.

Shared IP risk: Bulk SMTP services use shared IP pools across many customers. A customer on the same shared IP who sends spam can affect deliverability for other customers — the same risk exists with any shared IP, but bulk SMTP services have less active monitoring incentive than Microsoft's enterprise business.

Cold email policy: Many bulk SMTP providers explicitly prohibit cold email or require opt-in lists in their terms of service. Violations can result in account suspension.

No inbox reputation: Bulk SMTP services do not build individual inbox reputation through warmup. Each send goes out from the shared IP pool without the per-inbox reputation history that dedicated inbox warmup creates.

Option 3: Self-hosted SMTP server (not recommended)

Self-hosting an SMTP server (Postfix, Exim, or similar) gives maximum technical control but requires server administration, IP management, and active blacklist monitoring for the server's IP addresses. This approach is appropriate for large-scale email operations with dedicated technical teams. For B2B cold email at 1,000–100,000 contacts per month, the operational overhead is not justified by the control advantage.

SMTP vs. IMAP: what each does

Cold email senders using Instantly encounter both protocols:

When connecting an Inframail inbox to Instantly, both SMTP (outgoing) and IMAP (incoming) credentials are used. Instantly needs IMAP access to detect when a prospect replies (to pause the sequence) and to participate in warmup network email exchanges (to generate positive engagement signals by opening and replying to warmup emails).

What "custom SMTP" means in sending tools

Some cold email tools advertise "custom SMTP" as a feature. This means the tool allows you to connect any SMTP-compatible email account rather than requiring you to use the tool's own built-in email service.

Instantly supports custom SMTP connections: connect Microsoft 365 inboxes provisioned via Inframail, Google Workspace inboxes, or any other SMTP-compatible inbox. The "custom SMTP" feature is what enables Instantly to manage sequences from inboxes hosted on the sender's own domains rather than from Instantly's own email addresses.

This is the correct architecture for cold email: the sender controls the inbox, the domain, the reputation, and the warmup history. The cold email tool (Instantly) manages the sequence logic and connects to the sender's inboxes via standard SMTP.

Authentication and SMTP: how they interact

SPF, DKIM, and DMARC records (per the Mailgun SPF, DKIM, and DMARC guide) authenticate the SMTP connection at the domain level. When an email is sent via SMTP from a Microsoft 365 inbox:

• SPF verifies that Microsoft's servers are authorized to send email from the sending domain
• DKIM adds a cryptographic signature that the recipient's server can verify
• DMARC specifies the policy for handling SPF/DKIM failures

Authentication operates at the domain DNS level and does not change the SMTP protocol. Both authenticated and unauthenticated email uses the same SMTP protocol — authentication adds verification layers on top of the delivery mechanism.

Inframail configures SPF, DKIM, and DMARC automatically during inbox provisioning, so the SMTP connection from provisioned inboxes is fully authenticated without manual DNS work.

SMTP deliverability: what actually matters

When practitioners ask about SMTP and deliverability, they are usually asking: "Will my cold emails reach the inbox?" The answer depends on:

1. Domain reputation (built through warmup, monitored in Google Postmaster Tools)
2. IP reputation (the sending infrastructure's track record with mailbox providers)
3. Authentication (SPF/DKIM/DMARC passing)
4. Sending behavior (volume within safe limits, low complaint rate, low bounce rate)
5. Contact quality (Quarvio verified contacts to keep bounce rates near zero)

The SMTP protocol itself is not a deliverability variable. SMTP is the delivery mechanism; the factors above determine whether what SMTP delivers reaches the inbox or the spam folder.

Per Instantly's cold email benchmarks, top-quartile cold email senders achieve 15–20% reply rates. None of that performance difference is attributable to SMTP configuration. All of it is attributable to domain reputation, authentication, warmup, contact quality, and copy.

What practitioners say

"I spent two weeks researching SMTP configurations before our first cold email campaign and came away more confused than when I started. The breakthrough was realizing that SMTP is just the delivery vehicle and the real decisions are which inbox provider to use and how to warm up the domains. Once we got off a bulk SMTP service and onto dedicated Microsoft 365 inboxes through Inframail, our inbox placement went from around 50% to 90%+ within six weeks. The SMTP protocol was the same in both setups. The IP reputation and warmup were what changed." — G2 reviewer, Inframail reviews on G2

Instantly holds a 4.9/5 rating from 2,800+ verified reviews on G2, with custom SMTP inbox connection, warmup network, and deliverability monitoring cited as the infrastructure features that make dedicated inbox cold email performant without requiring direct SMTP management by the sender.

Our actual stack

Frequently asked questions

Does SMTP configuration affect cold email deliverability?

Not directly. SMTP is the universal email delivery protocol used by all email regardless of provider. Deliverability is determined by the reputation of the IP addresses used to send the SMTP connection, the domain reputation built through warmup, authentication records (SPF, DKIM, DMARC), and sending behavior (complaint rate, bounce rate, sending limits). The SMTP protocol itself is identical across providers.

What is the difference between SMTP relay and a dedicated inbox for cold email?

An SMTP relay service (like Mailgun or SendGrid) receives your email via SMTP and forwards it using the service's IP infrastructure. A dedicated inbox (like a Microsoft 365 inbox provisioned via Inframail) is a full email account on your sending domain, which sends email using the inbox provider's IP infrastructure and builds its own sending reputation through warmup. Dedicated inboxes are better suited for B2B cold email because they build per-inbox reputation, support ongoing warmup, and give the sender control over the domain and inbox lifecycle.

Why does Instantly ask for SMTP credentials when setting up an inbox?

Instantly connects to each sending inbox via SMTP (to send email) and IMAP (to receive email, detect replies, and participate in warmup). The SMTP credentials are the inbox account's outgoing email settings — username, password, and SMTP server address. Inframail provides these credentials during inbox provisioning. Instantly uses them to send sequences from the inbox on the sender's behalf.

Can I use bulk SMTP services like Mailgun for cold email?

Mailgun is technically compatible with cold email tools that support custom SMTP. However, Mailgun and similar bulk services are designed for transactional email and many prohibit cold email (unsolicited outreach) in their terms of service. More practically, bulk SMTP services lack per-inbox warmup capability and use shared IP pools that may carry different reputation characteristics than Microsoft 365's infrastructure. For B2B cold email, dedicated inbox provisioning via Inframail is the more appropriate infrastructure choice.

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The SMTP protocol is the same everywhere. Contact quality is not.

Inbox placement depends on domain reputation, warmup, and authentication — all of which can be set up correctly. The variable that collapses performance even on a correctly configured infrastructure is bounce rates from unverified contacts. Quarvio delivers pre-verified B2B contacts so the SMTP connection reaches a valid inbox every time. One-time purchase, no subscription.

Start your order on Quarvio →