The Mailserver tries to be as flexible as possible while maintaining security. All supported configurations require that connections between the mail client and the Mailserver are encrypted.
The following are supported configuration options for clients when connecting to the Mailserver.
| Protocol | Unencrypted port | TLS Port | SSL Port |
|---|---|---|---|
| IMAP | | 143 | 993 |
| POP3 | | 110 | 995 |
| SMTP | | 587 | 465 |
Port 25 is normally blocked with Greylisting, and it's not uncommon that ISP's block outgoing port 25 either, but otherwise port 25 could be used for SMTP. With port 587 with TLS or 465 with SSL, none of these problems exists though.
Short answer: Use IMAP!
Longer answer: POP3 is an older protocol that doesn't support folders. If you use POP3, the view in the mail client will differ from the Webmail as the Webmail use IMAP. Spam Learning is another feature that doesn't work from POP3 as it relies on folders for Detected Spams, and the Learn as spam and learn as no spam folders.
The only reason why POP3 is still available is that migration from old mailservers to the Mailserver would be difficult as many mail clients may already be configured for POP3.
As listed on the http://wiki.dovecot.org/POP3Server page as well, Dovecot is primarily an IMAP server.