Smtp

more info needed on problem, Does the users SMTP change randomly or is it something you want to do, does it affect all users or just one?

Now I am intrigued, Please explain more, How and where does the address change ? Is it in the exchange GAL or is the reported 'Reply-to' address changing on outgoing messages, Have you noticed anything else unusual happening, Is it always the same users or different ones each time, please add anything else you may think useful.

afterthought; If it is the 'Reply-to' address then the Magistr.b virus has the capability to change the reported 'From' name on messages as they are sent
This from McAfee:
[extract]
"An additional item of note is that this worm often alters the REPLY-TO
email address when mailing itself to others. In a similar fashion to the
other name changes made by this virus, one letter of the address is
incremented ordecremented. Thus when attempting to contact the infected
user to alert them, the message is often returned do to this address
modification."
[end extract]

look here for more detail

http://vil.nai.com/vil/virusSummary.asp?virus_k=99040

delete any spaces that appear in the url

This may be worth checking first.

Chris