For the benefit of others on the list, I should explain the problem:If you go to http://www.DNSreport.com and enter your E-mail address in the "Mail Test" box, you'll see the problem.Our email server doesn't receive emails from some email servers and receive from others. I encountered this problem with 2 mail servers till now. When sending from these domains i don't receive mail delivery fail message, though i don't receive it !! Can anybody help?
There are 2 MX records for the domain in question (which is good). The MX record with the higher priority is set up in the format "example.com. MX 10 mail.example.com.", which is good. However, the backup mailserver is listed in the format "example.com. MX 15 192.0.2.162". That says that the mailserver is a host named "192.0.2.162", which isn't a valid hostname (IPs and hostnames are very different beasts). The RFCs require the MX record to have a hostname, not an IP address.
So, in this case, most E-mail goes to the primary mailserver, which works fine. But if the primary mailserver is down or not reachable for some reason, the mail may end up bouncing, since the backup MX record is bogus. Due to socket programming commands, some mailservers won't know that an IP is there, and will be able to send the mail anyway (against the RFCs). But others (ones that follow the RFCs correctly) will not try sending to the backup mailserver, since the secondary MX record is bogus.
-Scott
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