IMAP on a mailserver, at least 3 ports are usually opened
25 smtp : incoming emails from anybody (whole internet) 465 smtps : outgoing emails from authorized users (to the whole intenet) 993 imap : imap for authorized users
I would like to configure postfix, so that authorized users can only send email through 465. By default this is not so. Users can also use STARTTLS over port 25. I would like to disable that.
My plan is to use port 25 for the public sending me email
use port 465 for my users (I can use firewall to allow specific IP ranges, or use custom port)
This would prevent port 25 being exploitable from brute force attacks, where hackers try to guess user/password. Port 25 simply would not accept user/password, even if it were valid. And since port 465 is restricted by firewall, hackers cannot exploit 465 either.
Is this possible in Postfix?
I am using Postfix 2.9.6-2 on Debian Wheezy