auth with entire email addr instead of just username

I am trying to configure a new IMAP based mail server. I thought I had everything correct. I have dovecot logging turned up and see that dovecot is processing the entire email address as the username like this... Oct 2 14:52:20 servername dovecot: auth: Debug: pam(testuser@domain.com,1.2.3.4): lookup service=dovecot ... and of course pam is reporting that as an invalid user. Moments ago when all was mbox, authentication worked fine. I can see Thunderbird has the username correct (no domain). What is tacking on the domain and how do I fix? Thanks in advance.