What is the difference between SMTP, POP3, and IMAP?

Still have a question, spotted an error, or have a better explanation or a source we should cite?

SMTP (Simple Mail Transfer Protocol) is the protocol that sends email. POP3 (Post Office Protocol version 3) and IMAP (Internet Message Access Protocol) are protocols that retrieve email from a server to your device.

Here's the practical difference: SMTP is what happens when you hit send. Your email client (like Gmail, Outlook, or Fastmail) uses SMTP to hand your message to a mail server, which then uses SMTP again to pass it along until it reaches the recipient's server.

POP3 and IMAP are what happens when you check your inbox. POP3 downloads your messages to one device and typically deletes them from the server (though you can configure it to keep copies). IMAP leaves everything on the server and syncs across all your devices. Read an email on your phone? IMAP marks it read on your laptop, too. POP3 doesn't.

Why this matters if you're sending email (not just using it): Most ESPs abstract all of this away. When you send a campaign through Mailchimp or Postmark, you're not touching SMTP directly. The platform handles it. But if you're building custom sending infrastructure, integrating with an API, or troubleshooting delivery, you'll hear "SMTP" constantly. It's the protocol your sending code actually uses.

You might also run into SMTP if you're connecting a custom domain to send through Google Workspace or Microsoft 365, or if you're using a transactional service like SendGrid or Mailgun via SMTP relay instead of their API. In those cases, you'll configure SMTP credentials (server address, port, username, password) so your app can hand off messages.

POP3 and IMAP matter less for senders, but if you're running a support inbox or shared team email, IMAP is what keeps everyone's view in sync. POP3 is mostly legacy at this point. Most modern email clients default to IMAP because nobody wants their phone and laptop showing different inboxes.

One last thing: SMTP is also the protocol spammers abuse, which is why authentication exists. SPF, DKIM, and DMARC are all built on top of SMTP to prove you're actually allowed to send from your domain.

Contributors

Who worked on this answer

Every name links to their profile. Every company links to their site. Real people, real accountability.

Yanna-Torry (YT) Aspraki

Founder · Author

Review My Emails

Ask an AI · tailored to your setup

Get specific help with SMTP, POP3, or IMAP

I read this on the Email Almanac about "What is the difference between SMTP, POP3, and IMAP": "SMTP sends email. POP3 and IMAP retrieve it. Most ESPs handle SMTP for you, but if you're building custom sending infrastructure or integrating via SMTP relay, you'll need to configure SMTP credentials." Help me understand how this applies to MY specific situation. Based on what you share below, I need: 1. If using an ESP (Mailchimp, SendGrid, etc.): Do I need to think about SMTP at all, or does the platform handle it? 2. If sending via API vs SMTP relay: Which makes more sense for my use case, and what's the trade-off? 3. If using a shared inbox or support email: Should I use IMAP or POP3, and why does it matter for team collaboration? 4. If I see "SMTP error" messages: What do common SMTP error codes mean, and where do I start troubleshooting? --- My details (the more you share, the better the advice): - Email platform/ESP: [e.g. Mailchimp, SendGrid, Postmark, custom SMTP, Google Workspace] - Sending method: API, SMTP relay, ESP dashboard, or not sure - Use case: [transactional emails, newsletters, support inbox, internal notifications] - Current challenge: [e.g. "seeing SMTP authentication errors", "deciding between API and SMTP", "team inbox not syncing"]

Edit the yellow boxes, then send to the AI of your choice.