Try to start Windows in safe mode with networking enabled. - Win10 http://windows.microsoft.com/en-us/windows-10/change-startup-settings-in-windows-10#v1h=tab01 - Win8 http://windows.microsoft.com/en-us/windows-8/windows-startup-settings-safe-mode# - Win7 http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/en-US/kb/safe-mode

Does the problem go away?

Running Vista. In any event, re-started both Windows and Thunderbird in safe mode, and yes the problem went away. Able to send/receive attachments w/ SSL/TLS enabled. What does that mean?

Windows safe mode disables anti-virus software, which is most likely the culprit.

What is your anti-virus software anyway?

Make sure your anti-virus software does not intercept your secure connection to the server.

Running Norton anti-virus. I thought of disabling Norton, and it didn't make a difference. However, since starting things in safe mode, the flow of attachments is working, albeit slowly on the download side. I appreciate your suggestions, and will continue to monitor and tweak the Norton settings. Right now, Norton is running fully, and Thunderbird has SSL/TLS enabled, and attachments can be sent and received (a bit slower than usual). Speed test numbers are normal.

Odd though that before I was getting "smtp.<servername> timed out" messages - as though it was a problem with the frontier server. The ping times were in the several hundred millisecond range, which I thought was too high.

See https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#Norton_Security

Interesting reading. It sounds like Norton will zap infected attachments even if SSL/TLS is disabled. So, maybe I'll try just leaving the SSL/TLS disabled. I wonder if auto-loading Thunderbird (v. 45) this week changed those settings to "enabled", and that's when the trouble started. Don't remember for sure which day this week I got version 45 of Thunderbird. Perhaps SSL/TLS was turned off before that, but I wasn't paying close attention before this started. Have you heard of version 45 changing SSL/TLS settings?

An update does not change account settings. Turning off SSL/TLS is a bad idea and not recommended.

The trouble may well have started due to a Norton update.

Well, SSL/TLS is now enabled (on both pop3 and smtp), and I can send/receive attachments again, and the download speed is back to where it should be. The ping numbers on pop3 and smtp are back to the 30-40 ms range again (normal).

To summarize - Thunderbird settings are the same as when I posted my first message here (SSL/TLS is enabled), and I don't believe I actually changed any Norton settings. After reading your Norton link, I clicked a few anti-virus email settings, but thought I put everything back the way it was.

The thing that seemed to put things right again (able to send/receive attachments with SSL/TLS enabled) was AFTER the reboot into safe mode and running Thunderbird in safe mode. Not sure how that alone fixed the problem, but it seemed to make a difference.

Thanks!