Domain name lost for SASL

Hi, I'm trying to set up Postfix + Dovecot (2.0.9) + PostfixAdmin on CentoOS (the standard amazon machine image) and am having problems with my SMTP user authentication. Turning on auth debugging I can see the username is losing the first part of the domain so

one@two.three

becomes

one.three

dovecot: auth: Debug: sql(one.three, ... ): query: SELECT username AS user,password FROM mailbox WHERE username = 'one.three' AND active='1'

I don't know if it's postfix or dovecot which is trimming the username, any tips on what causes this would be greatly appreciated.

Thanks