This was such a strange issue that I don't know if this post will ever help anyone. I just need to write it for therapy......
Starting on Friday of last week, I got reports from one client that some emails were not being delivered. This has happened to clients in the past when they were on block lists. So, my first step was to check some block list tools to see what's up. Each of the block list tools indicated that the IP was not being blocked.
Some of the antispam appliances have block lists that are not checked by the typical web sites. So, I tried to dig into it a bit further. One of the sites that was being blocked is a local university and I know several of the server/email admins there. So, emailed, explaining that I figured it was an antispam issue. They sent me the site to check block list for their appliance and it came up clean. In fact, they said my request for delivery wasn't showing up in the logs at all.
I should also note that access to ports other than 25 was unaffected.
You could do ping the IP of the mail servers and connect to websites on
that IP if a website was there. Only port 25 connectivity was blocked.
One of the things I had tried to do was use Putty (a free telnet client) to telnet to some of the failing email servers. Often the email server provides a message that indicates which antispam product is being used so that you can investigate why you're being blocked. In this case, each of the attempts to connect ended with a dropped connection.
Of course my other concern is that the firewall has gone nuts and is blocking things that it shouldn't. So, I did some logging and testing of rules. Nope, didn't look like the firewall was blocking anything.
As the suggestion of the admin at the university, I called the ISP. The admin at the university indicated that they had a call from someone else on Bell that couldn't deliver to them also. Weird....
So, next step is call the ISP. In the past my experience with ISPs and tech support has been rather poor overall. In this case Bell was great, they took the info and began investigating. They didn't blame my configuration. They also came back and indicated that it was their connectivity through Shaw that was the issue. So, anything being delivered to Shaw or routed through the Shaw network was affected.
It took about 5 days for Bell to convince Shaw that the issue was on their network, but the problem is now resolved. My best guess is that Shaw was marking the traffic from Bell like they do consumer Internet tracking and blocking port 25.