We have a business in town that sends different users emails. They're occasionally getting bounce back messages with the following errors: '550 4.4.7 QUEUE.Expired; message expired' and '421 4.4.0 Remote server response was not RFC conformant'. I spoke with our spam filter provider and they don't show the emails hitting the spam filter and said that their domain is doing a PTR look up before sending the mail to recipient end. Since the the mx record of our domain points to the spam filter server and does not match up to their A records it is sending the mail out to the MX record.
Not all of their emails are being returned, just occasionally. I'm hoping to get some more information so I can pass it on to them if it is their problem. I know they said they were getting returned emails from other businesses that they send to so it could be on their end.