Ok, if mail.*.com accepts SMTP authentication, then the problem boils down to getting Postfix to authenticate to mail.*.com as that user. What does mail.*.com use? SMTP with TLS? SMTP over SSL?
For postfix you need a tutorial like this one. This is just an example of relaying to gmail via SMTP over TLS, you will have to read many such tutorials to get all the points right.
Relay mail via Google SMTP with Postfix
It's highly unlikely that you can do this configuration from YaST, you'll have to get your hands dirty and edit main.cf