Whatever I do, with TLS, SSL, authentification, etc. or anything else neither Outook Express, nor Thunderbird can connect to your SMTP server. And it all started at the moment of your last upgrade.

I give up. I am sending my e-mails via web-mail now. Let's see if I can stand it. If I can't, I'll have to change the hosting (at least for my web-stores). I am sorry, guys. I really like your features, attitude and efforts, but the problems like this one explain pretty well why the other hostings are more expensive..

Ok, it eventually works!
It works with port 250, but not 25. It looks like the connection to port 25 is blocked somewhere on the way.

Everything is ok for me now, but couple of points by the way:
1. cPanel's email configuration file sets Outlook and other mail clients to use port 25 for SMTP. It would be nice if it set up port 250. I wouldn't ask that many questions if the autoconfig file was able to set up the connection

2. When I select "TLS if available" in my Thunderbird, the certificate about some "server.justedge.net" comes. Not that it was too important, but I somehow expected the certificate about "something.t35.net".

3. Is it possible to somehow subscribe to the notifications about major T35 upgrades? Maybe some RSS news feed?
I use a number of non-standard options: bigger number of addon domains, a bit bigger space than usual accounts have, a number of CNAME entries, etc. Every time you have upgrades I have problems. If I was notified in advance, I would be able to prepare a list of my special requirements in advance