I'm stuck in a classic customer dilemma and either Yahoo or Comcast is at fault, but they are currently pointing the finger at the other and refusing to budge.

I use Yahoo servers for my business email, and suddenly last night the SMTP server stopped working. I can download email from the POP server, but I can't send through the SMTP server. If I use the Yahoo web interface, I can send without a problem. Interestingly this morning, by business partner had exactly the same problem, and he also is on Comcast but on a different loop. Since we've had problems with Yahoo email in the past, I called their small biz support first and they did some mumbo-jumbo on their end and swear up and down that everything they are responsible for is fine and Comcast must be doing something to block access.

OK, so then I called Comcast. They are swearing up and down that they don't do any blocking that would affect a specific SMTP server and that Yahoo is the problem. Interestingly, it was right after this call my business partner found he could start sending email through Yahoo again.

So here I am, stuck. Everything seems to be working fine, except for access to this one SMTP server, which seems to suggest Yahoo is to blame. However, when I run traceroute to try and figure out if there is something between me and that server, I get nothing back. Traceroute can't seem to find any domain, even Google or LQ. To me, that suggests Comcast is the problem. I've rebooted my entire network and all the computers on it (Linux, Mac and Windows) have exactly the same symptom. They can do everything except email through this SMTP host.

Any ideas on how I can nail the culprit or am I doomed to pound my head on my desk in frustration until unconsciousness sets in?

I would guess that its Comcast blocking port 25. They like to do that to reduce SPAM, not that it would really help that much. Grab a copy of tcptraceroute and run 'tcptraceroute YAHOOMAILSERVER 25'. This will look simply like a tcp connection, but will tell you where it's getting hung up.

Why not run your own MTA, that way you in full control

Because Comcast blocks port 25 incoming.

Well, the good news is that the problem has vanished. The bad news is I'm no closer to finding out why it happened in the first place.

@Matir
That tcptraceroute is a nice little tool, and I think it is implicating Comcast. If I run it, I can see my gateway/router as the first hop, but then I get nothing but asterisks until the final destination appears. This happens for any site/port that I try. I had my business partner run it from his home, and he got real data back, so I'm starting to think that Comcast is messing with the loop that I'm on. However, I'm not sure that Comcast is blocking port 25, at least in this case. When I talked to Comcast, they pretty much said that they only worried about spam originating on Comcast, and if I had been using a Yahoo server as a relay, they would have come after me, not shut down access to a Yahoo server.

@coolb
Believe me, I've thought about running my own mail server. It really boils down to the fact that I have a limited number of hours to do things, and since running a mail server doesn't bring in dough, I need to offload that to someone else.