Activate 'force tls' for specific sending domain (not receiving)?

[K F]

Hi All
I'm trying to figure out how I can set up force tls for specific sending 
domains. I have several domains going through the postfix, and one of them has 
a requirement that TLS should be forced, the others are ok with May.And if it 
can't connect with tls, it should bounce the mail.
But I can't figure out how to do this for outgoing mails?
Best regardsKenneth