auth with entire email addr instead of just username

[Cliff Hayes]

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 at 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.