I have a location served by Comcast. I have a cable modem with an Apple Airport Extreme router. The IP address provided by Comcast is 22.214.171.124. Several computers are on the network. Most use Mac OSX Lion. But, we also have OSX Snow Leopard and Windows XP on Parallels.
About a month ago, all OSX Mail clients using smtp.comcast.net and smtp.gmail.com would send email to some addresses and the recipients never received the e-mail. There are no error messages. Email to Gmail, comcast, yahoo, etc. are delivered. Email to users at companies that own their own domain like tfarm.com are not delivered.
Eudora on Windows XP worked perfectly all the time.
If I take a Mac laptop to another location served by Comcast, the email works perfectly. I even brought a Mac laptop from another location to the "bad" location, and its email did not arrive at the recipient. So, it is something specific to the IP address at the bad location: 126.96.36.199. We have had the IP for a long time even though it is a dynamic IP.
This is a normal home that does not send any spam.
What on earth could be going wrong? Is it an smtp mail spam filter?
Now, here is some extra info. I removed the router and connected one Mac directly to the cable modem. This forced a new IP address. Miraculously, email worked perfectly. So, I swapped the Airport Extreme with another I had. This forced a new IP. Guess what, the email works perfectly.
So, the problem is tied directly to the IP address and I guess Mac email client. Can anyone explain what might be going on. Comcast was no help.
It has to be an IP block combined with the Mac email headers I think.