This article explains the most commonly used Email protocols on the internet – POP3, IMAP, and SMTP
- SMTP 25, 2525
- SMTP-SSL/TLS 587,465
- IMAP 143
- IMAP-SSL/TLS 993
- POP3 110
- POP3-SSL/TLS 995
587 vs. 465
These port assignments are specified by the Internet Assigned Numbers Authority (IANA):
Port 587: [SMTP] Message submission (SMTP-MSA), a service that accepts submission of email from email clients (MUAs). Described in RFC 6409.
Port 465: URL Rendezvous Directory for SSM (entirely unrelated to email)
Historically, port 465 was initially planned for the SMTPS encryption and authentication “wrapper” over SMTP, but it was quickly deprecated (within months, and over 15 years ago) in favour of STARTTLS over SMTP (RFC 3207). Despite that fact, there are probably many servers that support the deprecated protocol wrapper, primarily to support older clients that implemented SMTPS. Unless you need to support such older clients, SMTPS and its use on port 465 should remain nothing more than a historical footnote.