What's the difference between IMAP and SMTP?

SMTP sends outbound mail and relays between servers. IMAP retrieves inbound mail from a mailbox server while keeping copies on the server. They serve opposite directions — every full-featured email client uses both: SMTP to send, IMAP (or POP3) to receive.

Is SMTP or IMAP better?

Neither — they do different things. You need SMTP to send mail and either IMAP or POP3 to receive it. Modern setups use IMAP because it syncs the same mailbox across multiple devices while keeping messages on the server. POP3 is the older alternative that downloads and typically deletes from the server.

What's the difference between POP3 and SMTP?

POP3 retrieves mail from a server (downloading and typically deleting it) while SMTP sends mail out. They serve opposite directions. POP3 is older and now mostly used in legacy or low-bandwidth environments. IMAP has largely replaced it for modern client setups because it supports multi-device sync.

Do I need SMTP, IMAP, and POP3 all at once?

You need SMTP plus one retrieval protocol — either IMAP or POP3, not both. IMAP is the modern default. Most mail providers offer all three on different ports so you can pick what your client supports. Sending and receiving are independent — you can run SMTP without IMAP if you only send, or IMAP without SMTP if you only receive.

What ports do IMAP and SMTP use?

SMTP uses port 25 for server-to-server, 587 for authenticated client submission with STARTTLS, and 465 for implicit-TLS submission. IMAP uses port 143 (plaintext or STARTTLS) and 993 (implicit TLS). POP3 uses port 110 (plaintext/STARTTLS) and 995 (implicit TLS). For modern setups, use the TLS ports (465/993/995).

IMAP vs SMTP: What They Do and When You Need Each

The IMAP vs SMTP question usually comes up when someone is configuring a mail client or building an application that sends or reads email. The short answer: SMTP is for sending; IMAP is for receiving. POP3 is the older alternative to IMAP. You almost always need at least one of each.

This guide covers the practical difference, when each is appropriate, and the configuration details that catch people off guard.

SMTP, IMAP, POP3: what each does

Protocol	Direction	Purpose
SMTP	Outbound	Send mail from a client or relay between servers
IMAP	Inbound	Read mail on a server (keeps messages, supports folders, syncs across devices)
POP3	Inbound	Download mail to a device (typically deletes from server)

SMTP and IMAP/POP3 are independent. You can run one without the other depending on whether you need to send, receive, or both.

SMTP in detail

SMTP (Simple Mail Transfer Protocol) is defined in RFC 5321 and handles two distinct flows:

Server-to-server relay (MX delivery). Your sending server contacts the recipient domain's MX server to deliver mail. Port 25.
Client submission. A mail client (your app, a script, Thunderbird) submits a message to an SMTP relay for forward delivery. Ports 587 or 465.

These are the same protocol but different operational contexts. See SMTP port numbers explained for which port to use when.

SMTP is stateless from a delivery standpoint: once a message is accepted, there's no "outbox" or sent folder maintained by SMTP itself. The client (or sending server) handles tracking.

IMAP in detail

IMAP (Internet Message Access Protocol, RFC 9051 for IMAP4rev2) is a server-centric retrieval protocol. The mailbox lives on the server; the client browses and reads it remotely.

Core capabilities:

Multiple folders (INBOX, Sent, Drafts, custom)
Message flags (read, unread, flagged, deleted)
Partial message retrieval (download only headers, or specific MIME parts)
Server-side search
Multi-device sync (every client sees the same state)

IMAP is what you want if you check email from your phone and your laptop and expect the same read/unread state on both. Every modern hosted email service (Gmail, Microsoft 365, iCloud, Fastmail) offers IMAP.

POP3 in detail

POP3 (Post Office Protocol v3, RFC 1939) is the older alternative. The default behavior:

Client connects to the POP3 server
Client downloads all new messages to local storage
Client deletes the messages from the server
Client disconnects

Variants exist where the server retains messages, but POP3 fundamentally treats the server as a transit point, not the source of truth.

POP3 is still used in:

Legacy environments
Low-bandwidth scenarios where keeping mail on the server is expensive
Some single-device setups where users want a true local-only archive

For sender-side configuration, POP3 is rarely the right choice unless a specific app demands it.

Practitioner note: If a vendor's mail integration tool only supports POP3 in 2026, that's a signal to question their development priorities. IMAP has been the practical default since 2005 — anything that hasn't added IMAP support in two decades probably hasn't kept up on much else either.

Default ports

Protocol	Plaintext / STARTTLS	Implicit TLS
SMTP (relay)	25 (server-to-server)	—
SMTP submission	587 (STARTTLS)	465
IMAP	143	993
POP3	110	995

For modern client configuration, use the TLS ports: 465 or 587 for sending, 993 for IMAP, 995 for POP3.

When you need SMTP only

Applications that send notifications, transactional mail, or campaigns and don't need to read replies:

A SaaS app sending password resets
An ecommerce platform sending order confirmations
A marketing tool blasting newsletters

In these cases, you submit via SMTP (often to an ESP like SendGrid, Mailgun, or Postmark) and don't need IMAP. Replies typically go to a separate inbox managed elsewhere (a help desk, a CRM, or a regular mailbox).

When you need IMAP only

Tools that read but don't send:

A monitoring script that watches for bounces or unsubscribe replies
An automation that pulls invoices from a vendor mailbox
An analytics tool that summarizes inbound support requests

Most of these read from a mailbox via IMAP and never need to submit mail.

When you need both

Anything that acts as a full mail user:

A mail client (Thunderbird, Apple Mail, Outlook)
An automation that responds to incoming mail
A two-way customer support tool
Your own mailbox setup for personal or team use

Configuring SMTP and IMAP for major providers

Provider	SMTP (submission)	IMAP
Gmail	smtp.gmail.com:465 (SSL) or 587 (STARTTLS)	imap.gmail.com:993
Microsoft 365	smtp.office365.com:587 (STARTTLS)	outlook.office365.com:993
Apple iCloud	smtp.mail.me.com:587 (STARTTLS)	imap.mail.me.com:993
Fastmail	smtp.fastmail.com:465 (SSL)	imap.fastmail.com:993
Yahoo	smtp.mail.yahoo.com:465 or 587	imap.mail.yahoo.com:993

Authentication is typically OAuth 2.0 for Gmail and Microsoft 365 (app passwords for legacy). See SMTP settings reference for the full per-provider configuration.

Practitioner note: Microsoft 365 disabled basic SMTP authentication for most new tenants. If you're configuring a legacy app that only supports username/password SMTP, you'll either need to enable OAuth, use SMTP AUTH on a per-mailbox basis, or front it with an SMTP relay that handles OAuth on your behalf.

Self-hosted SMTP and IMAP

If you run your own mail server, the canonical stack is:

Postfix for SMTP (both relay and submission)
Dovecot for IMAP (and POP3 if needed)

These are paired in virtually every self-hosted distribution including Mailcow, iRedMail, and Modoboa. See Postfix Dovecot setup guide for the canonical configuration.

Which one matters for deliverability

Deliverability work is almost entirely about SMTP-side configuration:

SPF, DKIM, DMARC authentication
Sender reputation
Bounce handling
TLS on outbound connections

IMAP and POP3 only matter for deliverability indirectly — for example, a misconfigured IMAP server that disables one-click unsubscribe via Sieve filters might cause complaint-rate issues.

If you're configuring SMTP and IMAP for a production sending setup and want help getting the authentication, ports, and rate limits right, book a consultation. I configure mail infrastructure for SaaS, ecommerce, and agency clients regularly.

Sources

v1.0 · May 2026