the variable relay_domains specifies with which domains the sending of emails is allowedd. AS far as I unerstand if we set it to relay_domains = $mydestination there is no faking of email addresses possible?
As long as the user has a valid username and password for SMTP-AUTH or is sending directly from the server (e.g. with webmail) or sends a mail to a recipient on the server he can use whatever sender address he likes. You cannot prevent the user from faking the sender address.
I inserted these entries additionally, as they seemed "logical" to me. Any problems with that?
delay_warning_time = 4h
maximal_queue_lifetime = 5d
what about these settings? too paranoid?
I'd try it for a few days and then check if too many valid emails have been rejected.