SMTP vs IMAP vs POP3: what's the difference?

Q: SMTP vs IMAP vs POP3: how do they compare?

| Feature | SMTP | IMAP | POP3 | | --- | --- | --- | --- | | Purpose | Send / relay mail | Retrieve mail | Retrieve mail | | Direction | Outgoing | Incoming | Incoming | | Where mail lives | In transit | On the server | On one device | | Multi-device sync | N/A | Yes | No | | Offline access | N/A | Cached only | Full (downloaded) | | Secure port | 587 or 465 | 993 | 995 | The key mental model: SMTP is the postal truck that carries letters between post offices, IMAP is a mailbox you read from wherever you are, and POP3 is taking your letters home and emptying the box.

SMTP, IMAP, and POP3 are the three core email protocols, and they do different jobs. SMTP sends mail and moves it between servers. IMAP and POP3 retrieve mail so you can read it: IMAP keeps messages on the server and syncs them across all your devices, while POP3 downloads messages to one device and typically removes them from the server. In short, every account uses SMTP to send, then either IMAP or POP3 to receive.

Quick Takeaways

SMTP is the sending and server-to-server transfer protocol; IMAP and POP3 are retrieval protocols.
IMAP stores mail on the server and syncs read status, folders, and flags across devices.
POP3 downloads mail to a single device and, by default, deletes the server copy.
Choose IMAP for multi-device access; choose POP3 only for single-device use or strict local storage needs.
All three should run over TLS — use ports 587/465 for SMTP, 993 for IMAP, and 995 for POP3.

What is SMTP?

SMTP (Simple Mail Transfer Protocol) is the standard for sending email. When you hit send, your mail client hands the message to an outbound SMTP server, which relays it — often through several mail transfer agents — until it reaches the recipient's mail server. SMTP is a push protocol: it only moves mail forward toward its destination.

SMTP does not fetch mail into your inbox. That is the job of IMAP or POP3. This split is why your email settings always ask for an outgoing (SMTP) server plus a separate incoming server.

Modern SMTP uses port 587 for authenticated message submission with STARTTLS, or port 465 for implicit TLS. Port 25 still exists for server-to-server relay but is widely blocked for end-user clients because of abuse.

What is IMAP?

IMAP (Internet Message Access Protocol) retrieves mail while leaving the authoritative copy on the server. Your mail client syncs with the server, so folders, read/unread status, flags, and deletions stay consistent everywhere you check mail — phone, laptop, and webmail all show the same state.

Because the server holds the mailbox, IMAP needs a connection to browse older mail, and your storage is bounded by the mailbox quota. It is the right choice for almost everyone today, since most people read mail on more than one device. IMAP uses port 993 for implicit TLS, or port 143 with STARTTLS.

What is POP3?

POP3 (Post Office Protocol version 3) downloads messages from the server to a single device and, in its default configuration, deletes them from the server afterward. Once downloaded, mail lives locally, so you can read it offline and it does not count against a server quota.

The trade-off is that POP3 was designed for one device. If you download mail to your laptop and it is removed from the server, your phone never sees it. Most clients can be set to "leave a copy on the server," but POP3 still does not sync folders or read status between devices. POP3 uses port 995 for implicit TLS, or port 110 with STARTTLS.

SMTP vs IMAP vs POP3: how do they compare?

Feature	SMTP	IMAP	POP3
Purpose	Send / relay mail	Retrieve mail	Retrieve mail
Direction	Outgoing	Incoming	Incoming
Where mail lives	In transit	On the server	On one device
Multi-device sync	N/A	Yes	No
Offline access	N/A	Cached only	Full (downloaded)
Secure port	587 or 465	993	995

The key mental model: SMTP is the postal truck that carries letters between post offices, IMAP is a mailbox you read from wherever you are, and POP3 is taking your letters home and emptying the box.

Which protocol should you use?

For sending, there is no choice to make — you use SMTP, and you should authenticate it and run it over TLS.

For receiving, pick IMAP if you read mail on more than one device, want webmail and your phone to stay in sync, or rely on server-side search and backup. Pick POP3 only when you use a single device, deliberately want mail stored locally rather than on the provider's server, or work with very limited server storage. For most businesses and MSP-managed clients in 2026, IMAP is the default and POP3 is the exception.

What ports and encryption do SMTP, IMAP, and POP3 use?

Always prefer the encrypted variant of each protocol so credentials and message contents are not sent in plaintext.

SMTP submission: port 587 with STARTTLS (preferred) or port 465 with implicit TLS.
IMAP: port 993 with implicit TLS (preferred) or port 143 with STARTTLS.
POP3: port 995 with implicit TLS (preferred) or port 110 with STARTTLS.

These protocols move your mail, but they do not by themselves prove who sent a message. That is the role of authentication standards layered on top — SPF, DKIM, and DMARC — which verify the sending domain and stop spoofing.

Common issues with SMTP, IMAP, and POP3

Why does my mail send but never arrive?

If SMTP accepts a message but recipients never see it, the problem is usually authentication or reputation, not the protocol. Confirm your domain publishes valid SPF and DKIM records and that DMARC passes, then check whether the receiving provider quarantined the mail. Run the Email Security Score tool to spot missing records.

Why do my devices show different mail?

This is the classic sign of POP3 on a multi-device setup: one device downloaded and removed messages the others never got. Switch the account to IMAP so the server stays authoritative and every device syncs to the same state.

Why do I keep getting connection or timeout errors?

Check that you are using the correct secure port (587/465 for SMTP, 993 for IMAP, 995 for POP3) and that TLS is enabled. Many networks and ISPs block port 25 for client submission, which breaks SMTP if a client is misconfigured to use it.

Why is my mailbox full when I use IMAP?

Because IMAP keeps mail on the server, you are bounded by the mailbox quota. Archive or delete old messages, or increase the quota. POP3 avoids this by storing mail locally, but at the cost of multi-device sync.

Frequently asked questions

Can I use IMAP and POP3 at the same time? Technically some servers allow it, but mixing them on one account causes confusing state — one protocol may remove mail the other expects. Pick one retrieval protocol per account.

Is POP3 less secure than IMAP? Neither is inherently more secure; both must run over TLS (ports 995 and 993). The bigger security surface is authentication of the sender, handled by SPF, DKIM, and DMARC, not the retrieval protocol.

Does SMTP receive mail? No. SMTP only sends and relays. Incoming mail is delivered to the server, and you read it with IMAP or POP3.

How do these relate to MX records? An MX record tells other servers which host accepts inbound SMTP for your domain. SMTP then delivers to that host, where IMAP or POP3 serves the mail to users.

For MSPs and IT teams, the protocol choice is straightforward — IMAP for almost every client — but the harder work is proving those messages are authentic. Palisade automates SPF, DKIM, and DMARC management so client domains reach enforcement and stay there; the free Email Security Score shows exactly what is missing.