POP3
also: Post Office Protocol version 3, POP3S (POP3 Secure), POP (older versions)
POP3 (Post Office Protocol version 3) is an application-layer protocol used by email clients to retrieve messages from a mail server, typically downloading them to the local machine.
POP3 is a mail retrieval protocol that operates on TCP port 110 (or 995 for encrypted POP3S). When you configure an email client like Thunderbird or Mutt to check your email, it connects to a POP3 server, authenticates with your credentials, and downloads messages to your computer.
Unlike IMAP, POP3 is a simple, stateless protocol designed primarily for downloading and removing messages from the server. By default, messages are deleted from the server after retrieval, making POP3 ideal for users who access email from a single device. The typical workflow is: connect → authenticate → list messages → download → delete → disconnect.
Example: A Linux user might configure their mail client with pop3.gmail.com:995 as the server, which downloads their Gmail messages locally. Once downloaded, the emails exist on their computer and are removed from Gmail's servers.