[exim] Strip domain on SMTP Authentication

Hello,

I have successfully installed Exim 4.50 on a FreeBSD server. As well
SMTP authentication using just the username to authenticate.

I need to use username@??? to the SMTP authentication
mechanism. I know I need the adjust the server_condition, but I'm not
familiar enough with this.
Can anyone suggest me how can I perform this ?

Thanks for any help.
EL