I still don't understand which problem you are trying to solve
Are you aware that nowadays many mail installations use port 587 (with TLS/SSL) for their users to submit mail? Users have to authenticate with username and password an an encrypted channel is being used. For example gmail
Code:
Outgoing Mail (SMTP) Server - requires TLS: smtp.gmail.com (use authentication)
Use Authentication: Yes
Use STARTTLS: Yes (some clients call this SSL)
Port: 465 or 587
BTW, I am not a postfix expert