To diagnose problems with Thunderbird, try the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

Thanks but I had already tried to restart with everything disabled without success.

was that safe mode? "everything disabled" is not safe mode unless you actually used it. Disabling add-ons is only one thing that safe mode does. Although probably the most important.

Yes Matt I am using "Restart with Add-ons Disabled". I don't actually use any add-ons, plug-ins or extensions, and the default appearance is disabled by safe mode, but it will not connect to the email server.

Is it possible to roll back to the previous version of TB?

WHat anti virus are you using? We often see the firewall in internet security suites block Thunderbird post update. Norton's comes in for more than it's fair share of mentions for doing this.

Dam. I'm using Norton. What should I look for there?

The app is Norton Internet Security.

I have not used Norton for some 20 years. So exact details are not available to me, but check the firewall component that Thunderbird is allowed and the exact version number. My understanding is the the firewall is picky on version numbers. so for example 45.3 will work and 45.4 will not. For that matter 43.3.0 will work but 54.3.1 trips it up.

Thanks for that pointer. There was a conflict caused in Norton/Settings/Antivirus/Scans&Risks/Email Antivirus Scan/Configure. Both "what to scan boxes" were unchecked. I checked both and problem went away.