All I ever do when the problem occurs is restart Thunderbird and then it goes back to working for days or months. I do not compact my mailboxes in response to the problem so if the uncompacted mailbox caused a problem, I don't see why restarting would fix it.

My mailboxes compact according to the default Thunderbird settings. I think I see one of those - would you like to compact messages around once every week or two.

The first few times I reported the problem, people did ask me to compact and perform similar measures but there was no way to tell if they did anything for the issue. One person asked to do these things and I did, but then I asked if it's common to have to take measures like this just a few weeks after an install of TB prior and he seemed annoyed at me for not telling him that. I guess people assume that problems like this must be due to massive amounts of old data, though apparently it takes hardly any.

I have not seen the problem for several weeks and I have no idea why it sometimes goes for long periods like that. It's nice when it does of course, but it makes it extra hard to relate to any specific events or changes.

I have had all the mail folders excluded from AV scanning since the beginning. And as I mentioned before, I've seen similar threads on a variety of platforms and AVs and at least one thread that said no AV - so while I think excluding the folders is a good thing, I don't believe it's related to this issue.

I keep my mail and my mail server settings in my user directory - basically just a preference and a habit from earlier mail clients going back to the 90s. In my server settings directory I do have an Inbox and a Trash which haven't been modified since 4/17/2014 - probably the day I installed TB. They are both zero bytes. The only file in that directory with a recent access time is popstate.dat. In my main mail directory, I see the same mail folders that I see within Thunderbird. (Including Inbox and Trash files with access times of today and non-zero sizes.)

In my C:\Users\Dave\AppData\Roaming\...\mail server directory I have the same stuff as in the mail server directory in my user directory except that there is no popstat.dat file here. (ie just zero byte inbox and trash with 4/17/2014 modification dates).

I THINK those answer your questions.

I don’t use the smart mailbox feature.

Well, it has been two months and I haven't experienced this issue again. Obviously everything about this is anecdotal, but it seems like compacting my individual mail server inboxes (which may not be an option when using the Global Inbox configuration) may have made a lasting difference. Those files are still relatively small even now, and I have been prompted to compact a few times (presumably my Local Folders and not the mail server inboxes).

That having been said, I reviewed the previous post again and realized I left it unanswered. First off, I was asking about the C:\Users\<User>\AppData\Roaming\Thunderbird\Profiles\<default>\Mail directory and not a specific mail server directory. I am operating under the assumption that you are using default directories, and otherwise you would need to look wherever your mail servers are configured to save data (or perhaps were configured to save data prior to the Global Inbox configuration). My folder is like this: Local Folders mail.server1.com pop.server2.com pop.server3.com pop.server4.net pop.server5.tld pop3.server6.com smart mailboxes

This is the folder structure even though I never used smart mailboxes, and prior to my last action, some of those individual mail server inboxes were MBs instead of kBs in spite of the fact that they were empty. When I checked this morning, they were all below 100 kB in spite of receiving larger e-mails than that, so that seems like a good sign. Incidentally, my A/V is only excluding Local Folders instead of this entire Mail directory, but again, everything about my configuration is anecdotal, and 2 months without an issue may not really be that long (or I may have even experienced the issue at some point during that timeframe and not given it a second thought, but it is certainly not frequent like it was before my last post).

Thanks for the update. I am no longer using Thunderbird myself. I just ran out of time - I had family PCs to deliver, and didn't want to have to support this issue remotely, so I put us all on a different email client. Hopefully what you're doing will eventually lead to a fix. I'll check the TB release notes now and then, and if they ever find/fix it, I may give it another try.

Turns out the other client is very nice too - what was keeping me from trying it was that it has no direct GPG integration, but in the end, I decided it was easier to write up a little cut-n-paste encryption tutorial than it was to deal with a bug with random timing.