What TCP port is used for POP3 over SSL/TLS?

The TCP port used for POP3 over SSL/TLS is 995.

POP3S, the conventional name for POP3 carried over implicit TLS, establishes an encrypted connection immediately on TCP port 995. This protects login credentials and message traffic while they travel between an email client and the POP3 server. Although the name refers to SSL, modern implementations use TLS; SSL is the older predecessor and is obsolete for secure Internet communication.

The alternative is STARTTLS, implemented in POP3 with the STLS command. In that mode, the client initially connects to the ordinary POP3 service on port 110 and asks the server to upgrade the existing connection to TLS. Thus, port 995 identifies the implicit-encryption style, while port 110 can support encryption only when both sides successfully negotiate it.

The wrong options represent neighboring email services. Port 993 is IMAP over TLS, port 465 is traditionally associated with implicit-TLS SMTP submission, and port 587 is the standard message-submission port commonly upgraded with STARTTLS. POP3 itself is primarily a retrieval protocol, typically downloading messages rather than synchronizing a mailbox in the way IMAP does.