[CentOS] postfix login problem

Dear Friend,

I have postfix mail server. I open my mail server and put my username = jha and password= 123456 it is giving my error "*ERROR*Unknown user or password incorrect."

and In a log it is showing

-- Sep 20 13:03:22 neumann dovecot: imap-login: Aborted login (auth failed, 1 attempts): user=<jha>, method=PLAIN, rip=10.112.1.2, lip=192.168.1.2, secured

but When I put username = jha@XXX.ac.in so it is allow me to login in mail server .

Why it is taking full Domain Name .

I want like gmail and yahoo when we open it they just ask for username and password not full domain name.

Please help me..

Thanks & Regards Jitendra Jha +91-7498370550