File menu > compact folder will fix the issue in the affected folder. Probably a sync issue, or even a corruption caused by contention issues on the disk with other software. The date is the real give away. 1 January 1970 is the date value 0 in the C and C++ languages as this was picked by early unix engineers as a start date for time. Generally when it appears in your mail corruption of some sort is not far away and compacting generally fixes this, although at the expense of any corrupt emails.

Thanks for the suggestion Matt. In itself, the "File / Compact" option does remove the invalid entries that appear, as well as some duplicates that appear when Thunderbird first starts and does a batch download of emails from overnight.

The fact that duplicates persist to appear, and the occasional "Epoch" dated email would suggest some underlying problem still persists. I did a clean install a while ago (uninstalling and removing all the residual data), but the problems popped up again straight away anyway. Given that various accounts (GMail & Hotmail) were hoarding thousands of emails, which Thunderbird dutifully downloads, I have been cleaning them out, and maybe another clean install will fix the issue.

my guess is the problem keeps coming back because you have an anti virus or other system activity on your device that is causing file content6ion when Thunderbird opens and closes files. This can be scanning, backups, cloud synchronization or a host of other things. Most common are scanning of new incoming email and removing one that Thunderbird is trying to write to disk. WE do not recommend scanning of incoming email or on access scanning of the Thunderbird profile folder. Scanning of attachment opening should be sufficient and this occurs when the file is written to the temp folder before opening. As Thunderbird does not support scripts in emails they are not really a risk as they can not execute any malicious payload.

A more remote possibility is a failing drive with corruption at a low level, but you issues are reporting really do not point that way.

Since I removed the thousands of old emails from various accounts, at least for the last couple of days, there has not been any duplicates or "epoch" dated emails. Which is encouraging, but I'm not going to consider the issue as closed from my perspective until at least a few weeks have passed.

The only anti-virus software is the one "baked into" Windows 10. So I would hope that it is not causing any impact on Thunderbird. Most Phishing and other odd emails end up being deleted prior to opening anyway.

Nor, I hope that there is a drive problem. The drive is only a few months old, having mirrored an HDD to an SSD (mainly for noise and performance benefits).

Thanks for the suggestions Matt. I'll keep them under consideration should there be any further issue.

First thing this morning when the Thunderbird started, along with the rest of the computer, duplicate emails appeared in the Inbox again for one of the email accounts. Compact folders does de-duplicate but eventually an "Epoch" dated email appeared as well. So I did another clean install. Interestingly, only when the third email account was added to Thunderbird, did duplicates appear in that account. This reminded me that previously, duplicates had started to appear after the third email account was added to Thunderbird.

It can't have anything to do with the database sizes, "global-messages-db.sqlite", which certainly sounds like where Thunderbird stores its Inbox was 55Mb, the current one is 1.2Mb. And I am pretty certain the any media corruption shouldn't be an issue as the previous install databases are still there, just renamed so Thunderbird would ignore.

I suspect something else is afoot here.

For perspective.

I have 19 email account in this profile. Not all of them work, some have never worked but most do. They exist for testing ideas and causing issues that thunderbird has traditionally done badly (disabled/closed accounts.)

I have a global.messages.db.sqlite of 4.4 Gigabytes.

I found this profile struggled on my old system with 8 GB of ram but has no such issues with 16Gb of ram.

Mail is stored in the mail and IMAPmail folders. One folder per account named after the mail server. Each Thunderbird folder is a file in the file system. I try and explain that here https://thunderbirdtweaks.blogspot.com/p/blog-page.html

Well, it was only a guess that "global.messages.db.sqlite" contained the mail. But, hey, I learnt something about the data storage of Thunderbird. But as to why the problem floats to the surface, apparently, after the 3rd email account is added is still a mystery :(

So, all I need to do is wait until someone else has the same issue and draw a correlation there. Maybe there is a dependency on the date of the code initialising the databases? Who knows at this stage?