I've upgraded my IMP so I could add Kronolith I belive it was and have found an issue with using identities since then. If I create a "new" identity in IMP for the user, it always sends the mail as user@primary.domain, where as if I just edit the "default identity" for the user it will send the mail with user@virtual.domain. I did not note this problem while using a stock install of 5.5u2. While trying to fix it earlier today I have totally pooched IMP and will probably just upgrade to 5.6 to rectify. Just a heads up as you may be creating a new identity for the user and it may not be working correctly for you.
Terry