>A SWAG, but do you have reverse DNS set up at your site? It's not that widely used (yet) but some mail servers reject mail if you don't have it set up (or at least it counts as a demerit, and if a message has too many demerits it'll get rejected).
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>Are you able to send regular/conventional e-mails to those "problem" addresses? If so, how does the sending process differ between regular mails and your app?

Al,

Yes, I'm thinking it is something like not having reverse DNS (not that I know what that is exactly since I'm not really an IT guy) or some other demerit type situation.

Yes, we can and do resend these failed emails and attachments to these customers directly through Outlook from my desktop and they all go through.

I think the automation application sends the emails to a local SMTP server and then that sends or relays to our Exchange 2003 server and out the door.