IMAP, POP3 and SMTP are the three protocols behind email. SMTP sends mail. IMAP and POP3 read it: IMAP keeps mail on the server and syncs it across your devices, while POP3 downloads it to one device. A normal mail account uses SMTP for outgoing mail plus either IMAP or POP3 for incoming mail. This guide compares the three protocols, shows when to pick IMAP or POP3, lists the ports and security modes, and explains how the pieces fit together when you set up an email account or an application.
Email protocols at a glance
| SMTP | IMAP | POP3 | |
|---|---|---|---|
| Full name | Simple Mail Transfer Protocol | Internet Message Access Protocol | Post Office Protocol, version 3 |
| Direction | Outgoing | Incoming | Incoming |
| Main job | Send and relay mail | Read and manage mail on the server | Download mail to a device |
| Mail stored | Passes through; not a mailbox | On the server | On the device, often removed from the server |
| Typical ports | 587, 465 (submission); 25 (server to server) | 993 (TLS), 143 (STARTTLS) | 995 (TLS), 110 (STARTTLS) |
| Specification | RFC 5321 | RFC 3501, RFC 9051 | RFC 1939 |
How the three protocols work together
Imagine you write a message to a friend. Your email app hands it to your provider's outgoing server using SMTP. That server finds your friend's provider using DNS and delivers the message to it, again over SMTP. The message lands in your friend's mailbox on that provider's server. When your friend opens their email app, it uses IMAP or POP3 to fetch the message from the mailbox. SMTP carried the message, and IMAP or POP3 collected it. No single protocol does both jobs.
Our guides to SMTP and IMAP go into each in detail, and the article on the mail transfer agent shows what happens in the middle.
IMAP vs POP3: which should you choose?
Choose IMAP if you read email on more than one device, if you want sent mail, drafts and folders to stay in sync, or if you want your mail to be available on the web. IMAP is the default for most providers today.
Choose POP3 if you read mail on a single computer, want messages stored locally, and have limited server storage. POP3 also suits older systems and some devices that only support it. If you use POP3, check whether it deletes mail from the server after download, because that decides whether other devices can still see it. Many clients offer a setting to leave a copy on the server for a number of days.
| Your situation | Better choice |
|---|---|
| Phone, laptop and web mail | IMAP |
| Shared mailbox used by a team | IMAP |
| One old computer, local archive, small server quota | POP3 |
| Device that supports only one protocol | Whichever it supports |
Where SMTP fits when you read mail with IMAP or POP3
When you set up an email account manually, the app asks for two servers. The incoming server is IMAP or POP3. The outgoing server is SMTP. They often have different names, such as imap.example.com and smtp.example.com, and different ports. A common mistake is to enter the incoming server name for both. If you can read mail but cannot send, check the outgoing settings first. Read SMTP settings, ports and authentication for what each field means.
Security for all three
All three protocols were designed before encryption was everywhere. Modern use needs TLS on every connection. Use implicit TLS ports where available: 993 for IMAP, 995 for POP3 and 465 for SMTP submission. Or use the STARTTLS ports (143, 110 and 587) with encryption required. Never use a plain-text option with a real password. RFC 8314 explains why implicit TLS is the preferred approach. Read email SSL and TLS explained for the details and how to test.
Many providers now require app-specific passwords or OAuth for IMAP, POP3 and SMTP, because ordinary passwords are easy to steal. If your login fails with a correct password, check that.
Other ways to read and send email
- Webmail uses a browser. You do not need IMAP or SMTP settings, because the web application talks to the server itself.
- Exchange ActiveSync and similar proprietary protocols are used by some corporate systems.
- Email APIs let an application send mail with an HTTP call instead of SMTP. See SMTP or API.
- Inbound webhooks deliver incoming mail to an application as an HTTP request instead of through IMAP. See inbound email processing.
Common mix-ups
- "My app can receive but not send." The IMAP or POP3 settings are right and the SMTP settings are wrong.
- "My sent mail does not appear on my phone." With IMAP, the app must save sent messages to a Sent folder on the server. With POP3, sent mail stays on the device that sent it.
- "I deleted mail on my phone and it is still on my laptop." This suggests POP3, or a client that is not syncing.
- "Which protocol sends newsletters?" SMTP, or an email API. Neither IMAP nor POP3 sends anything.
Rule of thumb: SMTP out, IMAP in. Pick POP3 only if you have a reason to keep mail on one device.
For applications that send email
If you are building an application, only SMTP, or an API, matters for sending. Use a hosted service so that authentication, retries and reputation are handled for you. See SMTP servers vs SMTP services and our SMTP relay.
POP3 in more detail
POP3, the Post Office Protocol version 3, is older and simpler than IMAP. After login, the client asks the server how many messages there are, retrieves them one by one, and may tell the server to delete them. The classic commands are USER and PASS to log in, STAT and LIST to see what is waiting, RETR to fetch a message, DELE to mark one for deletion and QUIT to end the session and apply the deletions. Because POP3 has no concept of folders on the server or of per-message flags, two devices using it cannot easily agree on what has been read. Some clients mitigate this by leaving copies on the server for a time. POP3 is specified in RFC 1939, and its encrypted port is 995.
A typical set of settings
These values show the shape of a complete setup. The names are placeholders, and your provider's help page has the real ones.
| Purpose | Protocol | Server | Port | Security |
|---|---|---|---|---|
| Incoming (recommended) | IMAP | imap.example.com | 993 | SSL/TLS |
| Incoming (alternative) | POP3 | pop.example.com | 995 | SSL/TLS |
| Outgoing | SMTP | smtp.example.com | 587 | STARTTLS |
A short timeline
SMTP appeared first, in 1982, to move mail between systems. POP followed in the early 1980s so that people with a personal computer could collect mail from a server. IMAP arrived in the mid-1980s to let people manage mail on the server from several places, and it grew in importance as people moved to webmail and mobile devices. The three have been revised several times, but their roles have stayed the same, which is why a setup from twenty years ago still looks familiar.
Setting up email for an app or a domain? Tell us what you need to send and read, and we will tell you which protocol and settings to use.
References and further reading
Frequently asked questions
What is the difference between IMAP, POP3 and SMTP?
SMTP sends mail. IMAP and POP3 read it: IMAP keeps mail on the server and syncs it across devices, and POP3 downloads it to one device and often removes it from the server.
Is IMAP better than POP3?
For most people, yes, because it keeps mail and its state in sync across phone, laptop and web. POP3 suits a single computer with local storage or devices that only support it.
Do I need SMTP if I use IMAP?
Yes. IMAP only reads mail. To send mail you need an outgoing SMTP server in the same account.
What ports do IMAP, POP3 and SMTP use?
IMAP uses 993 with TLS or 143 with STARTTLS. POP3 uses 995 with TLS or 110 with STARTTLS. SMTP submission uses 587 with STARTTLS or 465 with TLS.
Why can I receive email but not send it?
The incoming IMAP or POP3 settings are probably correct but the outgoing SMTP settings are wrong. Check the SMTP host name, port, security mode and credentials.