Adjust DNS settings for mail

Hello, I recently got a problem with my server: Some other servers would decline Emails sent out by my server giving that error: "550-This address no longer accepts mail. 550 Sender verify failed" As far as I found out this is since it´s not properly setup (either the server or the DNS) according to some RFC... So I need that corrected. I guess it should be only a few settings, but I don´t know them and therefor need some help on that. The server is based on linux, being administered by DirectAdmin.