Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

And reducing the size of your Inbox would be a good idea too.

Thank you for the excellent suggestion. Adding the file `<profile folder>\Mail\<acct name>\Inbox` to the AV exclusions list seems to have solved the problem. I did not have to exclude the entire folder, nor the associated `.msf` file.

It does concern me though that this is a security risk, given that email is a frequent vector for malware. Does the "Allow antivirus clients to quarantine individual incoming messages" still provide protection against this?

I'm not certain that this was a size issue, as I have another mailbox folder that is 3.1GB, is not excluded from AV, and has no slowness/freezing issues. Also, I wonder what changed in Nebula that surfaced this problem.

It does concern me though that this is a security risk, given that email is a frequent vector for malware.

For scanning of incoming messages, as Thunderbird does not execute scripts in the body of emails, getting malware is very difficult from opening a mail.

Getting malware from an attachment is a different issue and quite possible, but before an attachment can be opened, it has to be decoded (from MIME) and a copy placed in the systems temp folder for Thunderbird or the opening application to use. When that file is created it will be scanned by your anti virus product, or so they claim. So while there are some minor risk with incoming mail, you should not get infected by just mail opening.

Does the "Allow antivirus clients to quarantine individual incoming messages" still provide protection against this?

The idea is primarily to avoid anti-virus software corrupting Thunderbird mail files. But if traffic in these support forums is anything to go by, a very significant proportion of mail issues are actually caused by the anti-virus products that are supposed to be protecting you.

I'm not certain that this was a size issue, as I have another mailbox folder that is 3.1GB, is not excluded from AV, and has no slowness/freezing issues.

Inbox is most susceptible to corruption. The file changes every time a message is received. And your ant-virus software will scan your 1.5GB Inbox file every single time. In doing so it blocks Thunderbird to access it, and eventually corrupts it in the process to quarantine a supposedly harmful message. Other mail files may not frequently being being scanned by anti-virus software and hence are less likely to become corrupted.

Also, I wonder what changed in Nebula that surfaced this problem.

The problem is primarily anti-virus software, not Thunderbird. However, there was a problem in TB128 were compacting folders could cause corruption for IMAP accounts. This is supposed to be fixed in TB 128.3.3.

I appreciate the response and the solution. I respectfully submit that this is not *entirely* an antivirus problem. The other, 3.1GB mailbox file that I refer to, is also an `Inbox` for a different account. It is not excluded from AV scanning, and it works well.

Secondly, I have had the same mailbox files/folders for years and the problem between TB and AV appeared only after the Nebula upgrade. So while I agree that the freeze is not TB's "fault", I suggest that there is something in the new TB code that is causing adverse interactions with the AV software, that was not present in the previous version. I only raise this as something to be considered by the devs in the future, as a potential symptom of a bigger problem. This is a POP connection, not IMAP and I am already on TB 128.3.3.

Thank you for the perspective on malware infection via email, this is helpful. For now, I will continue to have the relevant file excluded from AV scanning but hope to have a better solution in the future.