Mail/DNS Problems

In response to a client getting bounced by AT&T and Comcast I have been working with their T1 service provider to set up reverse DNS records for their mail server. The problem is the static public IP address: the IP address that mail appears to be coming from is NOT the IP address of the mail server! Here specifically is what is happening: Let's say that the public IP of the mail server is 10.11.12.250. Using DynDNS I have set up the following A records: • bum.example.com maps to 10.11.12.250 (bum = BackUp and Mail in case you are wondering!) • mail.example.com maps to 10.11.12.250 • webmail.example.com maps to 10.11.12.250 But when I receive a message from the server, here is what the header says: [code]Received: from bum.example.com (ip10-11-12-248.z12-11-10.customer.algx.net [10.11.12.248])[/code] So why does it appear that mail is coming from 248 instead of 250?! Now 248 is within my assigned block of IP addresses, but it isn't assigned to anything anywhere from what I can tell. Any ideas on what could be causing this? Any ideas on how to troubleshoot this and figure out where the error is occuring? And, last but not least, is it going to be a problem if I just have the service provider create a PTR record for 248 that maps to bum.example.com? Any help would be greatly appreciated! -John