The problem is that when I sent mail, the received mail is from different user.
I sent mail to myself and the sender wasn't me, but one of mailboxes that is on my domain.
Are you sending from a DirectAdmin server? Then this shouldn't happen unless you've got the wrong settings in your local email account (on your machine or webmail) because DirectAdmin's mailservers (both exim and Dovecot) don't change the From address.
Thank you and sorry, it seems to be my mistake. I found out in the squirrel mail settings that the user name was somehow changed. I don't recall to make such a change so I was wondering how it was like that.