According to fastmail.com ("SSL/TLS vs plaintext/STARTTLS port numbers" part) and thousands of other sources in the Internet (emphasis mine):
- IMAP uses port
143, but SSL/TLS encrypted IMAP uses port
- POP uses port
110, but SSL/TLS encrypted POP uses port
- SMTP uses port
25, but SSL/TLS encrypted SMTP uses port
That doesn't count to morons who produced Microsoft Outlook 2010. They've decided that switching encryption method for IMAP between
SSL will change port from
993 and back (correct!). While, doing exactly the same for SMTP won't change port at all.
This way Outlook is problably the only mail client in the world who is trying (unless you manually correct it) to establish secure, encrypted SMTP connection over port 25. An attempt which is doomed by its definition. Congrats again, Microsoft fucks!