I upgraded my new MBP to Lion and now I can't send mail if I'm on my network at home. After looking at other threads (like this one: https://discussions.apple.com/thread/3192956) and based on the symptoms I'm having, I think the problem is likely an AT&T problem. When I use the Lion laptop on a different network, mail works and I can send mail with no problems. That suggests to me that the network is the issue.
I called AT&T to tell them this and they feel everything is working on their end so it must be my email client. They want to charge me to talk to tech support because I'm using an email client instead of their web interface.
Does anyone using AT&T have the same problem? Does anybody know how you get through to a big company like AT&T who isn't listening?
Believe it or not, sending mail from home seems to now be working for me. The strange thing is that when I made the change I'll tell you about, it didn't work at first, but now it does. The only explanation I can think of is that I've rebooted my machine since I made the change and that seems to have made the difference.
Anyway, here's what I did. In Mail, go to File > Preferences > Accounts. Now, select the account in the left pane if it isn't already selected. In the main pane at the bottom you should see an option for TLS Certificate. Mine had 'None' selected so I opened the combo box and saw there was a certificate I could choose (just a big long string of numbers and letters). I chose that, saved my changes, and that was it.
Like I said, nothing changed immediately after I did this yesterday. But this morning, I can now send mail. So either this change (and a reboot) fixed the problem for me or coincidentally AT&T fixed the issue on their side - I don't know.
Give this a shot and let me know what happens.