SMTP sends mail. IMAP and POP3 retrieve it. SMTP runs on port 587 (submission) or 25 (server-to-server). IMAP keeps mail on the server and syncs across devices (port 993). POP3 downloads mail and typically deletes the server copy (port 995). Modern setups use SMTP for sending and IMAP for receiving.
SMTP vs IMAP vs POP3: What Each Protocol Does
SMTP, IMAP, and POP3 are the three core email protocols. Every email exchange uses some combination of them. For senders, understanding which protocol does what helps you configure clients, debug delivery problems, and reason about authentication. This page covers what each does, default ports, and which to use when.
The short version: SMTP sends, IMAP reads (across devices), POP3 reads (single device, mostly obsolete).
SMTP (Simple Mail Transfer Protocol)
SMTP is for sending. It does two jobs:
- Client-to-server submission — your mail client uses SMTP to hand outgoing messages to your mail server. Port 587 with STARTTLS, authenticated.
- Server-to-server transfer — your mail server uses SMTP to deliver mail to the recipient's mail server. Port 25, no authentication beyond IP reputation and DNS-based checks (SPF, DKIM).
Client → SMTP submission (port 587) → Your mail server → SMTP transfer (port 25) → Recipient mail server
Defined in RFC 5321 (transfer) and RFC 6409 (submission).
SMTP ports
| Port | Purpose | TLS | Status |
|---|---|---|---|
| 25 | Server-to-server (MTA-to-MTA) | STARTTLS optional | Standard for server transfer, blocked on most consumer ISPs and cloud providers for client submission |
| 587 | Client submission | STARTTLS required | Modern default for sending from clients |
| 465 | Client submission (implicit TLS) | TLS from connect | Legacy but still common; deprecated then re-allowed by RFC 8314 |
For sender setup details, see the SPF setup guide — SPF authenticates the SMTP envelope sender during server-to-server transfer.
IMAP (Internet Message Access Protocol)
IMAP reads mail that lives on the server. The client never owns the mail; it's a view onto the server's copy. Multiple clients can connect simultaneously and see the same folders, read flags, and changes propagate.
Phone → IMAP (port 993) → Mail server (mailbox storage)
Laptop → IMAP (port 993) → Mail server (same mailbox)
Both devices see the same inbox. Marking a message read on the phone updates the flag on the server; the laptop sees it on next sync.
Defined in RFC 9051 (IMAP4rev2). Most servers still speak RFC 3501 (IMAP4rev1).
IMAP ports
| Port | TLS | Status |
|---|---|---|
| 143 | STARTTLS optional | Avoid — easy to misconfigure to cleartext |
| 993 | Implicit TLS | Standard, recommended |
POP3 (Post Office Protocol v3)
POP3 downloads mail to the client and (by default) deletes the server copy. It's older than IMAP and made sense when storage was expensive and people checked mail from one device.
POP3 is mostly obsolete in 2026. Some use cases still:
- Single-device users with strong local backups
- Migrating an old mailbox to a new provider
- Programmatic mail processing where the script owns the mailbox
Defined in RFC 1939.
POP3 ports
| Port | TLS | Status |
|---|---|---|
| 110 | STARTTLS optional | Avoid |
| 995 | Implicit TLS | Standard if you must use POP3 |
Practitioner note: I still see POP3 in the wild on legacy cPanel setups where the user has been on the same account for 15 years. Migrating to IMAP usually breaks their local archive (Outlook .pst files for mail that was POP3-downloaded years ago). When migrating, export the local archive separately before flipping to IMAP.
Side-by-side comparison
| Feature | SMTP | IMAP | POP3 |
|---|---|---|---|
| Direction | Send | Receive | Receive |
| Mail location | In transit | Server | Client (after download) |
| Multi-device sync | N/A | Yes | No |
| Folder support | N/A | Yes | No (most clients) |
| Read flags sync | N/A | Yes | No |
| Standard secure port | 587 | 993 | 995 |
| RFC | 5321, 6409 | 9051 | 1939 |
Where to find these in client setup
When you add an email account to a client, you'll typically configure:
Incoming server: imap.example.com port 993 SSL/TLS
Outgoing server: smtp.example.com port 587 STARTTLS
Authentication: username + password (or OAuth)
For the major providers:
- Gmail:
imap.gmail.com:993/smtp.gmail.com:587(app password or OAuth) - Outlook.com:
outlook.office365.com:993/smtp.office365.com:587 - iCloud:
imap.mail.me.com:993/smtp.mail.me.com:587 - Yahoo:
imap.mail.yahoo.com:993/smtp.mail.yahoo.com:587
See where to find email settings for the menu paths in each major client.
What senders need to know
For bulk and transactional senders, SMTP is the only protocol that matters directly. Your ESP exposes an SMTP relay (smtp.sendgrid.net, smtp.mailgun.org, smtp.postmarkapp.com) and you submit mail via SMTP submission or their HTTP API.
IMAP and POP3 are for your inbound side — the mailboxes you (or your recipients) use to read mail. They have no direct effect on outbound deliverability, but the protocols you offer affect how your users access their mail.
Practitioner note: If you're running your own mail server, support IMAP and SMTP submission as the defaults. POP3 is optional. Make sure both run on the TLS ports (993, 587, 465) only — disable cleartext IMAP and SMTP submission entirely.
Modern protocol replacements
JMAP (JSON Meta Application Protocol) is a newer alternative to IMAP, designed to be more efficient over high-latency networks. Fastmail and a few others support it. It hasn't displaced IMAP at scale yet.
For most senders and operators in 2026, the SMTP + IMAP combination is what you'll work with for the foreseeable future.
If you're standing up a mail server and want help with protocol configuration, TLS hardening, or IMAP vs POP3 decisions for legacy users, book a consultation. I do mail server setup and migration for small businesses moving off cPanel mail or onto self-hosted stacks.
Sources
- RFC 5321 — SMTP — IETF
- RFC 6409 — Message Submission for Mail — IETF
- RFC 9051 — IMAP4rev2 — IETF
- RFC 1939 — POP3 — IETF
- RFC 8314 — Cleartext Considered Obsolete — IETF
v1.0 · May 2026
Frequently Asked Questions
What's the difference between SMTP, IMAP, and POP3?
SMTP (Simple Mail Transfer Protocol) sends mail from client to server and between servers. IMAP (Internet Message Access Protocol) reads mail kept on the server, synchronized across devices. POP3 (Post Office Protocol v3) downloads mail to a single device and usually deletes it from the server. Modern users want SMTP + IMAP, not POP3.
Should I use IMAP or POP3?
IMAP, almost always. IMAP keeps mail on the server so it's available on every device, supports folders and flags, and survives device loss. POP3 only makes sense for single-device users with strong local backups or anyone with very limited server storage. Most modern providers still offer POP3 but recommend IMAP.
What ports do SMTP, IMAP, and POP3 use?
SMTP: 25 (server-to-server, often blocked), 587 (submission with STARTTLS, recommended), 465 (implicit SSL, legacy but still supported). IMAP: 143 (cleartext, avoid) or 993 (SSL/TLS, standard). POP3: 110 (cleartext, avoid) or 995 (SSL/TLS, standard).
Is SMTP only used for sending email?
Yes. SMTP exclusively handles message submission and transfer. To read mail, you use IMAP or POP3 (or webmail, which uses the same protocols server-side). When you 'send a reply,' your client uses SMTP to send the new message and IMAP to update the read flag on the original.
What's SMTP submission and why does it matter?
SMTP submission is the authenticated form of SMTP used between mail clients and outbound mail servers, defined in RFC 6409. It runs on port 587 with STARTTLS and requires authentication. The 'plain' SMTP on port 25 is for server-to-server transfer only and shouldn't accept unauthenticated client mail.
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