IMAP and POP3 are mailbox-access protocols. They are not the protocols used to transfer mail between Internet mail servers.
IMAP is designed around server-side mailbox state and synchronization. It supports folders, message flags and clients that leave the authoritative mailbox on the server.
POP3 is a simpler retrieval model historically associated with downloading messages to a client.
Common ports
| Protocol | Cleartext / STARTTLS | Implicit TLS |
|---|---|---|
| IMAP | 143 | 993 |
| POP3 | 110 | 995 |
Modern deployments should protect authentication and message content with TLS. Many hosted services also favor modern token-based authentication over long-lived mailbox passwords.