For some reason, when certain people try to send us email to our domain, they get a bounce back. There are very few that actually are having this problem.
mail.parasec.com is one mail server that cannot send to us. AOL, GMail, and hotmail can all send fine to our domain.
Our domain name is mail.montanacorporate.com. I have run a dnsstuff.com DNS report, and everthing comes back with a pass. I have a reverse PTR record setup. I had a SPF record setup, but deleted it to make sure it was not causing a problem. I have made sure the banner on my Exchange server reads mail.montanacorporate.com. I have made sure we have both a mx record and a host record. the mx is setup with a priority of 10 and there is only one.
I have also made sure that there is nothing checked on the smtp server like apply sender id, or apply inteligent message filter. All are unchecked.
There is no AV installed on the server.
I have no idea what to do to get this fixed.