Virii that can exploit email server?

[Joshua Thomas <JThomas () poweronemedia com>]

Is anyone aware of a virus, transmitted via email, that is able to exploit
the email server it resides on? 

Eg: 

User A is infected by virus
User A sends email to friend
Virus follows email and is stored on Email Server A
Virus exploits Email Server A, infects other emails, causes world
domination, etc.

As far as I know this is not possible because the MTA should treat the virus
as data, not code. Have there been any known cases of the above? I have done
some research on Symantec's virii database, asked around, etc. and haven't
heard of anything.

Thanks,

Joshua Thomas
Network Operations Engineer
PowerOne Media, Inc.