Disable any "pop3-login" on Dovecot

If you don't use imap or pop3, then remove dovecot:

sudo aptitude remove dovecot dovecot-pop3d dovecot-imapd

Dovecot has nothing to do with your outgoing mail, you're probably using postfix, exim, or some other MTA for that.

If you want to use IMAP but not POP3, then just remove the dovecot-pop3d backage. If you want POP3 but not IMAP, just remove dovecot-imapd.