What are the settings? They should be smtphm.sympatico.ca on 587, STARTTLS, authentication = normal password, User Name = email address. What is the antivirus and its settings for scanning email? I would leave the mail.server preference at its default, as it's not the one that affects timeouts.

Hi - thanks for responding. I guess I wasn't clear that I'm not changing the settings in the config file - it happens automatically. I have changed nothing with regard to the account settings and yes, all my settings are as you suggest. I have also made no changes to the computer's security settings. Since the problem began, the only possible change is that there was an update to Thunderbird that happened automatically. Thanks again, Elizabeth

It frequently happens that either TB or the security app, or both, update and become incompatible. To test this, run the computer in Windows safe mode to bypass startup apps, then run TB. The security/AV app should always exclude the TB profile folder. Many of these apps interfere, and you would be better off with Windows Defender, with the TB profile added as an Exclusion in WD settings.

I tried rebooting into Safe Mode. I can recreate the issue in Safe Mode. I'm running Norton Security 360 and I've made sure the profile folder is excluded everywhere I can exclude things. The problem recurs. I tried turning off Norton entirely and that doesn't help either.

Any other suggestions?

Is there any way to step through what's happening to see which is communicating with what that's actually causing the configuration to rewrite itself?

Thanks again for your time.

Is there a problem sending from a gmail account, or is it just with the Sympatico account? If it's just with Sympatico, it indicates a problem with their server, instead of a TB setting. In that case, I can explain how to send from the Sympatico account through the gmail smtp.

It happens with both type of accounts. Or perhaps I should say all three as I have two gmail accounts, one bell account using pop and one using imap. I have no problems with these accounts on other mail apps. So my concern isn't that I can't communicate, so much as I really like Thunderbird and I don't want to give up on it, you see.

Plus, it's weird.

I would focus on the gmail accounts, since they should work if the settings are correct, as they do for literally millions of TB users. If a gmail account doesn't work, the problem is very likely related to the network or some external factor (antivirus, VPN, router software). Use imap.gmail.com on 993 or pop.gmail.com on 995, smtp.gmail.com on 465; SSL/TLS security, OAuth2 authentication, User Name = email address for both servers. If you want to test with a new profile, create one from Help/Troubleshooting, about:profiles. The existing profile is not affected, and can be launched also from about:profiles.

I'll give it a whirl and let you know, thanks.

Hi. I've been avoiding this because I am embarrassed to say, this problem was way more basic than I made it. I wasn't taking into account all the other systems we have knocking around the house these days and there was a conflict with the email on one of the laptops and the desktop. When I changed the port on the outgoing server on one of them the problem disappeared. It was equal parts satisfying and infuriating. Let it be a lesson that no matter how thorough one is in describing a problem there's always something left out.

I am so sorry to have troubled you. Thanks again!