What protocols make email work (SMTP, POP, IMAP)?
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You send email from your marketing platform or transactional service, subscribers read it on their phones or laptops. But how does a message you send from Klaviyo end up readable in someone's Gmail app? Three protocols make it work: SMTP (Simple Mail Transfer Protocol), POP3 (Post Office Protocol version 3), and IMAP (Internet Message Access Protocol).
SMTP is the sending protocol. It's how mail servers talk to each other to move messages across the internet. When you hit send in Mailchimp or Postmark, your ESP uses SMTP to hand off your message to the recipient's mail server. SMTP is a push mechanism: your server pushes the message to the next server in the chain until it reaches the recipient's inbox. Every email you send travels over SMTP, whether it's a password reset or a newsletter.
POP3 and IMAP are receiving protocols. They're how email clients (like Apple Mail or Outlook) pull messages from a mail server so the recipient can read them. POP3 downloads messages to the device and usually deletes them from the server. That's fine if you only check email on one computer, but if you read email on your phone and laptop, POP3 means messages downloaded on your phone won't show up on your laptop. (Most people stopped using POP3 years ago.)
IMAP keeps messages on the server and syncs them across all devices. Mark an email read on your phone, it shows as read on your laptop. IMAP is what powers modern multi-device email. Nearly every webmail service (Gmail, Yahoo Mail, iCloud Mail) uses IMAP under the hood, even if you never configure it yourself.
Why senders should care: you don't configure POP3 or IMAP (your subscribers do), but you absolutely configure SMTP. Your ESP handles the technical details, but understanding that email moves over SMTP helps you troubleshoot delivery. When an email doesn't arrive, the failure happens somewhere in the SMTP chain: your server couldn't reach the recipient's server, the recipient's server rejected it, or a relay in between blocked it. The path an email takes is entirely SMTP handoffs.
These three protocols are why email is universal. A message sent from SendGrid over SMTP can be read in Fastmail over IMAP, or in Tutanota with their own protocol. The protocols are open standards, which means any mail server can talk to any other mail server. That's the entire reason email still works 50 years later.
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