Mimemail returning Server address error

I am trying to send mail from my drupal site (on IIS) through an SMTP Server which is part of the company's Exchange Server.

I get the message:

warning: mail(): SMTP server response: 501 5.5.4 Invalid Address in F:\webspace\disk15\Newfreak\www\sites\all\modules\mimemail\mimemail.module

I have been everywhere and I just can't find a reason or a way to arrange that. I am supposed to launch my site at the end of this week and I am stuck with this problem which will postpone everything....

Any ideas? I thank everyone in advance.

Helio