Templates are not mail, and neither are draftes, simply be virtual of not being complete sent documents. In IMAP accounts folders are synchronized with the server, along with retention periods, so perhaps you need to check the server side settings as well as Thunderbird retention settings as the server side settings will override. If, say Thunderbird is set to not delete and the server has a retention period of 1 month, the server will win every time...

In our setup Drafts and Templates are real, and I've seen Thunderbird try to delete other items I know to be real.

The server side scripts that archive mail and does general housekeeping doesn't delete folders, it only affects mail in the folders.

I've gone on to the server and read the code for the scripts and they don't do anything to Drafts and Templates. I have mail going going back to 2014 in my Drafts folder, which is further confirmation that they aren't touching them.

I'm going to switch off the cache in Thunderbird and see if that resolves the problem, I'm just worried about the performance hit.

I switched off the cache and purged it. Also switched on hardware acceleration.

Now Thunderbird doesn't get stuck deleting folders, but it's still doing it. It keeps on trying to delete the Junk folder whenever I open Thunderbird (last 3 sessions).

Last session I checked which Junk folder it was trying to delete (I have 3 accounts for work), and it was the 3rd one. Checked on the mail server, that account doesn't have a .Junk folder, but it shows in Thunderbird. Not sure if it ever had one or if Thunderbird has finally managed to nuke it on the server. Company policy is to not use it, so that's not serious.

Just restarted Thunderbird. It just deleted the non existent Junk folder again in the 3rd account, but now the Junk folder icon is missing for that account too.

Also deleted the Template folder in my 1st acccount, but I don't have a Template folder on the server and haven't had one in a while. It doesn't show up in the Thunderbird folder list either.

Thunderbird tried to delete one of my monthly archive folders for 2018. That folder exists and there's nothing on the server side to do anything to that folder for another few years.

If Thunderbird actually manages to deleted one of the archive folders, that's it, game over. I'd have to go over to something else that isn't trying to delete my mail despite every setting I can set being set.

How do you know Thunderbird is trying to delete anything. You keep saying it is trying to delete folders, but where do you get this information from?

When you right click the folder and select properties from the menu what does the retention tab or the folder actually say?

And despite you assertions to the contrary, a draft is a work in progress and it is supposed to be deleted and replaced with the latest version very X minutes and finally deleted when you click send. If your company has a different policy, it is unfortunate that they do not understand what a draft is. Trying to reconcile reality to the unfounded misapprehensions of executives appears to be almost a full time job for IT professionals. (good thing I am not one)

In the status bar at the bottom and when I click Tools > Activity Manager.

The retention policy is "Use my account settings". My global account settings are: "Don't delete any messages"

My company doesn't have a different policy, I just used that as proof that Thunderbird isn't actually deleting anything. It's the only thing keeping me reasonably calm otherwise I'd be losing my mind.

Here a list of the folders that Activity Manager says Thunderbird has deleted in just this session (I don't leave Thunderbird running in the background):

Deleted folder...

2015-08 Sent-6 Sent Sent-5 Sent-2 Sent-1 Sent-8 Sent-7 Sent-4 Sent-3 2015-01 Sent-6 Sent Sent-5 Sent-2 Sent-1 Sent-8 Sent-7 Sent-4 Sent-3 2015-10 2015-03 Sent-6 Sent Sent-2 Sent-16 Sent-9 Sent-10 Sent-12 Sent-4 Sent-3 Sent-11 Sent-17 2015-12 Sent Sent-2 Sent-1 Sent-3 2015-06 Sent Sent-5 Sent-2 Sent-1 Sent-4 Sent-3 2015-05 Sent-6 Sent Sent-5 Sent-2 Sent-1 Sent-8 Sent-7 Sent-4 Sent-3 2015-02 Sent-6 Sent Sent-5 Sent-2 Sent-1 Sent-8 Sent-7 Sent-4 Sent-3 2015-04 Sent-6 Sent Sent-5 Sent-13 Sent-2 Sent-1 Sent-18 Sent-15 Sent-20 Sent-16 Sent-9 Sent-10 Sent-8 Sent-12 Sent-7 Sent-19 Sent-14 Sent-4 Sent-3 Sent-11 Sent-11 Sent-17 2015-09 Sent-6 Sent Sent-5 Sent-2 Sent-1 Sent-7 Sent-4 Sent-3 2015-07 2015-11 2012-12 2012-11 2012-08 2012-09 2013-01 2013-09 2013-05 2013-07 2013-11 2013-10 2013-02 2013-12 2013-06 2013-03 2013-04 2013-08 2019-03 Sent 2019-01 Sent 2019-04 2019-02 Sent 2019-05 2010-10 2010-05 2009-10 2009-07 2009-11 2009-08 2014-09 Sent 2014-06 Sent 2014-12 2014-07 2014-11 Sent 2014-02 Sent 2014-05 Sent 2014-10 Sent 2014-04 Sent 2014-08 Sent 2014-01 2014-03 Sent 2018-04 Sent 2018-11 Sent 2018-02 Sent 2018-03 Sent 2018-05 2018-06 2018-10 Sent 2018-12 Sent 2018-07 Sent 2018-01 Sent 2018-08 2018-09 Sent

This is all on my 1st account. No one else in my company has reported the same issue and my boss that has even more mail than I do.

Ok, so you are as far as I can see reporting on synchronization activity.

The following IS a bug in maildir, but I mention it as an example of IMAP synchronization. IF you right click a folder in an IMAP account that is configured to use the IMAP maildir like storage and select repair. The actual result is the folder is deleted and all mail and the folder is re-downloaded from the server. From the user chair what you see over time is Thunderbird is very sluggish on accessing the folder and an uptick in the network activity and then everything returns to normal.

That is a clear bug, as the full emails should not be downloaded over again, but based on the IMAP spec the contents of the headers should to rebuild the index that repair invokes.

So, why is email in the folder being deleted and re-downloaded. This can have many causes, the most common that I have seen is changes to the server mail program, upgrades, moves to new hardware etc. I really don't understand the nitty gritty. But if for any reason the server updates the index of the messages in the store then the mail Thunderbird holds is no longer "on the server"because the index is no longer valid so is deleted (synchronized to match the server index) and the content of the folder downloaded over again to bring the folder back into a synchronized state.

So I think your getting excited over nothing. But you have to be aware that your local copy is not an archive. It can be blown away by changes on the server. It is more of a local cache to speed access.

Not only was that all those "deletions" irritating, but it frequently stopped me from using Thunderbird as it made it unresponsive.

Using hardware acceleration and disabling the cache seems to have mostly made that less painful.

Previous attempts to get way from the problem by using a new profile didn't work. My latest attempt has worked, so now I have to go through the pain of documenting all my settings and set up the new profile without coping any files in the hope that the bug wont be copied over too.