[Dovecot] multiple Authentication

I'm using the following authentication passing the full email address user@domain.com

auth default { mechanisms = plain

# Linuxconf passwd-like file with specified location passdb passwd-file { args = /etc/vmail/shadow.%d }

# Linuxconf passwd-like file with specified location userdb passwd-file { args = /etc/vmail/passwd.%d }

user = root }

But - what I want to do is if the user doesn't enter the domain part I want it to assume one specific domain. How would I do that?