Curious email send requirements

I've only been using Virtualmin a short while, and am beginning to put it into production..

I've encountered an issue with sending emails..

When receiving i use user@domain.com however, when sending i have to use user-domain.com

When sending using user@domain.com i get: SASL LOGIN authentication failed: authentication failure in maillog

and in messages i get: saslauthd[2697]: do_auth : auth failure: [user=user] [service=smtp] [realm=domain.com] [mech=pam] [reason=PAM auth error]

any ideas why the @ version isn't working, and where to start looking to try and solve it.

BTW: I'm on a clean CentOS 5.4

Thanks

C