What Are Email Protocols? An Email Protocol Guide
Get an overview of the most important email protocols—SMTP, IMAP, and POP3—and how they work together to deliver your messages.
Email Protocol Basics
Email delivery depends on several standardized protocols rather than a single system. Outgoing mail is typically sent using Simple Mail Transfer Protocol (SMTP), while incoming mail is retrieved from servers using either the Post Office Protocol version 3 (POP3) or the Internet Message Access Protocol (IMAP).
Each protocol has a specific role: SMTP moves messages between servers, POP3 downloads mail to a single device, and IMAP keeps mail synchronized across multiple clients by leaving messages on the server.
SMTP, IMAP, and POP3 Compared
SMTP is responsible for pushing messages out: when you hit send, your email client uses SMTP to hand the message to your provider’s server, which then relays it to the recipient’s mail server. SMTP can also handle replies, forwarding, and error notifications.
For reading messages, POP3 and IMAP take different approaches. POP3 is designed to download mail to a single device and optionally delete it from the server, which made sense when storage and connectivity were limited. IMAP, by contrast, maintains folders and read/unread status on the server, making it ideal for accessing the same inbox from phones, laptops, and webmail.
Security and Configuration Considerations
Modern email setups almost always use encrypted versions of these protocols, such as SMTPS (SMTP over TLS) and IMAPS (IMAP over TLS), to protect credentials and message contents in transit. When configuring an email client, you will typically specify server hostnames, ports, and whether to use SSL/TLS.
Understanding the basics of these protocols helps you troubleshoot issues like duplicate messages, missing mail on certain devices, or warnings about insecure connections. It also provides context for related technologies such as SPF, DKIM, and DMARC, which build on email protocols to improve authentication and reduce spoofing.