postfix related to: Undelivered Mail Returned to Sender

[Ovidiu]

hello,

the problem is as folows:

one of the sites I hoste uses phpBB2 forum. It seems like they have set the sender address of their registration emails to an external yahoo account.

What now happens is this:

a user tries to sign up, gives a wrong email address, phpBB2 sends out a welcome message to the wrong email address, the target mailserver rejects the wrong emailaddress so after sitting in my queue the mail gets back to the server admin account, shoul't it get back to the sender email address set in the phpBB2 settings?

I mean the server has a message it can't deliver, so it should look at the sender of the mail and deliver the failure back to that email address or does this not work if the sender was also an external email, which would indicate relaying ?

can somebody give some hints?