What is your anti-virus software?

Avast program version 10.4.2233, Virus definition 151021-2

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-including-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?

Also see https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVAST

I tried windows xp safe mode, and had considerable trouble even connecting to a wifi network. Much less to Thunderbird. Anyway, I doubt that it's an antivirus problem, because I have another machine, Windows 7, with identical antivirus software (Avast and Thunderbird) and no problems.

What I really want to know is whether I can download a clean copy of Thunderbird without risking losing all my email history.

magnetix said

I tried windows xp safe mode, and had considerable trouble even connecting to a wifi network. Much less to Thunderbird. Anyway, I doubt that it's an antivirus problem, because I have another machine, Windows 7, with identical antivirus software (Avast and Thunderbird) and no problems. What I really want to know is whether I can download a clean copy of Thunderbird without risking losing all my email history.

NO, mistakenly I indicated that the action "solved the problem" BUT IT DID NOT. I do not think the problem is with anti-virus software. I think it's a corrupted Thunderbird, and I want to download a clear Thunderbird. How can I do that without risk?

Your data and personal info is stored separate from the program in your Profile. You can reinstall the program and not overwrite your profile. As with all computer operations it is always a good idea to make a backup to be safe and as a good practice you should make backup regularly. You can Google Thunderbird profile backup for that procedure.

I doubt that it's an antivirus problem

If you search this forum you'll find similar complaints and the culprit always was the anti-virus software. Starting Windows in safe mode was an attempt to confirm this. Without the confirmation you can try to create an exception for C:/DOCUME~1/Peter/LOCALS~1/Temp and for your Thunderbird profile folder, so that those won't be scanned by your anti-virus software.

As already mentioned by Airmail, personal info is stored in the profile and separate from the program installation. The kind of problems you mentioned are almost certainly related to the data in your profile, and not to a corrupted Thunderbird installation. If you want to re-install Thunderbird or use an older version, just go ahead. But I think it will be a waste of time. Furthermore, older versions do have unpatched security vulnerabilities. So you may actually open a can of worms here. Anyway, it's up to you.

I feel that I owe a reply to the Forum. The fault seems to mostly with Avast. I came up with the following fixes: - install latest update of Avast, which happens to have come out the day before. This is version 11.1.2241. The look of the user screen is quite different - to the Avast - Settings - exclusion list add "*\nsemail.html" (not including the quote marks). - to the Avast - Settings - Active Protection one could uncheck Mail Shield, but this might be too drastic. - however, under that same head, uncheck "Scan outbound mail" (because what's the point of checking your own mail?)

These fixes seem to cure the problems of - - double copies sent - - missing HTML - - missing body of email However, the output email can be quite slow to arrive at the receiving computer.

Still unfixed is the occasional problem with imap.googlemail.com:993 "confirm security exception". It may have something to do with START/TLS.

to the Avast - Settings - exclusion list add "*\nsemail.htm

Also exclude the Thunderbird profile folder.

what's the point of checking your own mail?

Exactly. Scanning of outgoing messages is completely useless.

Still unfixed is the occasional problem with imap.googlemail.com:993 "confirm security exception".

What is the exact error message you get?

Do you let Avast intercept your secure connection to the server?

Did you read the support article for issues caused by Avast? https://support.mozilla.org/en-US/questions/1089933#answer-798401

Here is what I get for the "security exception" incident: Add Security Exception You are about to override how Thunderbird certifies this site. Location: smtp.googlemail.com=465 The site attempts to identify itself with invalid information. Unknown identity. This certificate is not trusted because it hasnt been verified as issued by a trusted authority using a secure signature. My answer to this is: confirm The same message happens with imap.googlemail.com=993 This happens every couple of days. Sometimes there are up to 100 such messages.

For the third time: Did you read (and understand) the support article for issues caused by Avast? https://support.mozilla.org/en-US/questions/1089933#answer-798401