If the folder is corrupted beyond fixing by the Repair Folder step, it may help to rebuild the folder according to these instructions. Basically, you copy the existing messages to another folder, close TB, delete the mbox and .msf files for the folder, then TB automatically rebuilds the folder upon restart. With an IMAP account, messages will be downloaded from the server, but with POP accounts, deleting the popstate.dat will reset the status of downloaded messages.

I don't think it's necessary, or desirable, to disable McAfee real-time scanning, but you should definitely disable any options for scanning mail folders (add the profile folder as an exception or exclusion), spam detection or secure-connection scanning.

Thanks for the replies, I haven't deleted the mailbox yet as it seems a bit drastic to start with. Some people on the other similar post were critical of McAfee, so had a look at that, I found that somehow I had 2 McAfee apps, 1 from BT and 1 that was an unsubscribed version. So deleted the unsubscribed. Have also re enabled real time scanning and excluded the popstat file from scanning. Has run for 2 days now without issue, I will update later as it will need a week of clean running before I can be sure that its resolved. Thanks again for help

Still have same problem, have now added inbox.msf to files excluded from scanning. Repair folder works but seems to create a duplicate of the bad emails 1 correct and 1 still wrong. Inbox is minimal - only 3 or 4 current emails, but I keep past emails in TB folders. Would a reinstall of TB be the easiest solution? Would I lose all the data saved in my folders if I reinstalled?

Reinstalling the program won't fix a problem with the profile folder or the effect of McAfee. Plus, there's no benefit in excluding an index .msf file, as the messages are stored in the mbox file, which have the same name as the msf file, but no extension. Set the entire TB profile folder as an exception.

Thanks for the further suggestions - apparently you cant select a whole folder in McAfee, you have to select individual files.

If you can't exclude folders, and adding individual files is too onerous, you should disable all actions by McAfee on email, leaving the real-time background scanner on:

https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#McAfee

This seems 'fixed' at the moment - so thanks for all the suggestions esp sfhowes. I tried turning off ALL McAFee actions for a day or two just to make sure. All now re enabled except anti-spam and I haven't had a problem for over a week now (previously every day or two). So it looks like it was the McAFee anti spam that was doing it. Thanks again for all the support,.

Spoke too soon, problem came back today. Strange that it happens after such a long time - perhaps after a scheduled scan? Have now turned off all real time scanning. Followed the ling suggested by sfhowes above but it seems this refers to a different version of McAfcee, its not possible to add whole folders to the exclusion list - and theres too many files to add individually. If anyone knows which files to specifically add I'd appreciate knowing. Will also try contacting McAFee b ut Im guessing they're not interested as its a McAfee produst badged as BT Virus protect.

Well that was useless! McAfee can't/won't help as I dont have a subscription with them, they referred me to BT Virus Protect (even gave me the telphone number) but BT say they only help with installation not McAfee problems. I've turned McAfee off completely for a week - if that resolves the problem then looks like I need a new anti virus provider (and a new ISP).

I would remove McAfee entirely and use the built-in Windows Defender. Add the TB profile folder as an Exclusion in WD Virus and threat detection settings.

Uninstalled McAFee entirely, run for 6 days without problem. Contacted McAfee to find out if there was a fix, but they wouldn't talk to me as the product was branded as BT Virus protect. They gave me the support number in BT who were completely clueless (why am I surprised?) they only offer installation support and referred me back to McAFee. so a deadly embrace here, but McAfee did then send me an email asking what I thought of their support. I told them. Then received a call from McAFee who did explain that they were aware of the problem and releasing a fix that would automatically update, but they couldn't answer for BT virus protect. So logged the problem with BT. Have now installed latest version of Virus protect, if problem is still there I think I'll just unistall permanently.