Email & Messaging

TCP 25 carries SMTP mail transfer, commonly between mail servers. Mail clients should use the submission endpoint specified by their provider.

Post Office Protocol version 2 (POP2)

POP3 (Post Office Protocol v3) fetches mail over TCP 110. After downloading, whether server copies are deleted depends on the client DELE command and server policy — unlike IMAP (143/993), which is designed to keep copies on the server. POP3 is plaintext; the TLS variant is POP3S on 995.

IMAP on TCP 143 can upgrade a connection with STARTTLS when supported. It differs from implicit TLS on port 993.

Port 465 is used for mail submission with implicit TLS: the TLS handshake begins immediately after the TCP connection.

Port 587 is for message submission, commonly from a mail client to its provider. Distinguish it from server-to-server relay on port 25.

Port 993 provides IMAP mailbox access over implicit TLS. It is for reading and synchronizing mail, not SMTP submission.

TCP 995 is POP3S — POP3 wrapped in TLS from the first byte. It is the secure replacement for plaintext POP3 on 110 and the default in modern mail clients.