[imp] default identity for all users

[P. Mark Anderson]

Hello,
 
MY ISSUE:  default identity is the same for all users
VERSION:  IMP 3.2, with app authentication (imp)
 
There seems to be one and only one default identity for all IMP users.  
How do I configure it so that each user's default identity is that 
user's username at domain?  Right now, each user's reply address is that 
of the main default identity.
 
Thanks,
Mark