[imp] Problem with Domain Auto Detection

[geno@onshore.net]

Were currently preparing to move our production mail server from IMP3 to IMP4. 
I have got it working on another machine but I'm running into problems auto
detecting the domain the user is accessing.  Basically we have around 40
domains with webmail hosts that all resolve to the same IP.  We got IMP3 to
detect what domain the user was accessing and then would append it to their
user name when sending outbound email.  I was under the impression that when
you hide mail server options it would auto detect the domain and then do a
similar thing so that ie. user bob could access webmail.testing1.com and
webmail.testing2.com which both have entries in servers.php and due to the auto
detection when bob logins he would then appear to be bob at testing1.com or
bob at testing2.com.  For some reason I can't get IMP to distinguish between the
virtual domains and it just keeps bob at testing1.com even if his url shows
webmail.testing2.com.  Any ideas what I might be doing wrong?  I appriciate any
suggestions.