Do you have any Message Filters that say to perform a 'Delete Message' ? If you use Message Filters there is a 'Filter Log' button - click on it. If it's not enabled then select the 'enable' checkbox so it's useful to see what's filtering and where in the future. If it is enabled then it should give you a list of what has been moved deleted etc. That will tell you what filter (if any) is performing the delete. Then you can modify the appropriate message filter and -or selecting 'Move message to' and select the server Trash folder.

Thanks for the reply. Those were the first things I checked. I don't have any (active) filter and the account is set to "move to trash".

If no filters operating in Message Filters' and the Menu Bar toolbar says : View > Threads > 'All'

Check - Settings > Privacy & Security Junk Section Does it say move them to accounts junk folders or 'delete them' ?

If junk is set up ok.

This is beginning to sound like you may have an Anti-Virus which is removing emails. Check - Settings > Privacy & Security 'Security' section 'Anti-Virus' subsection Is this selected 'Allow Anti-Virus clients to quarantine individual incoming messages' ? Suggest uncheck it if it's selected.

Do you allow an 'Anti-Virus' access to scan anything in your Thunderbird profile folders ? I have set my Anti-Virus to stay clear of my Thunderbird profile as it's known they can cause issues. I have profile in default location so in 'Anti-Virus' set up to exclude this 'Thunderbird' folder (and any of its files and folders) from scanning. C://Users/username/Appdata/Roaming/Thunderbird

Hey Toad-Hall,

No quick filter: "Inbox 2 Messages" at the top left

Junk-Settings > Destinantion and Retention is unchecked 'Allow Anti-Virus clients to quarantine individual incoming messages' is unchecked I have explicitley excluded the Thunderbird/Profiles folder from the windows defender.

My activity log of TB explicitly contains twice something like "Deleted 356 items from Inbox". They are not moved to trash (as selected in the account settings)

What can cause this?

Is this a Pop or Imap mail account ?

If emails get deleted off the server then when Thunderbird connects to server - it will auto delete any emails that do not exist on server because Imap accounts can only display whatever is on server.

In webmail account accessed via abrowser do you have any filters or anything forwarding emails to another account?

Do you have any other device which accesses that account eg: a phone ? If it is using imap and you delete emails then they get moved off server and thus get deleted from an imap account. Or it uses a Pop connection which auto deletes off server when it connects and downloads. OR another email client which is installed on same computer but it is operating in the background ? I've found several people who had previously used Outlook and upon booting up computer it was still on an auto start, so was downloading mail to a pop account in the background and cause the deletion off server.

IMAP.

I was able to see a mail that was gone in the client for a few minutes still in the webmail. Also my it department showed me some logs which tells:

> Jan 29 18:48:44 > imap(....)<...><....>: Info: > delete: box=INBOX, uid=847, > msgid=<....>, > size=3773

(With a connection from "Thunderbird, version=128.6.0" and my ip adress)

Therefore I know it is a client issue.

There is an option in the account Synchronization & Storage settings to automatically delete messages (both locally and on the server) to save disk space. And each folder may have its own version of those settings in Properties > Retention Policy.

EDIT: This would affect old messages, not newly arrived ones, though.

This has to be a message filter… Did you check the Filter Log as Toad-Hall suggested or just that you don't have any active filters? Does manually deleting messages work as expected, i.e. do deleted messages really go to Trash and stay there?

Clarification: After I had lost all messages last week, I deleted the account and added it back again. Yesterday the same thing happened again, with everything in the account on default settings. Therefore no filters. The log was not enabled. Disk space: "Don't delete any" As a test: Manually deleting a mail sends it into trash. Retention policy of the folder: "Use account settings"

Also I noticed that the messages were deleted in two batches of ~300 mails, 10 hours apart.

Not sure how it could be related, but I've seen enough of these that may be worth a try…

Assuming this isn't being caused by a message filter, install an add-on to be able to see and change folder flags:

https://addons.thunderbird.net/thunderbird/addon/folderflags/

After installing it, in Thunderbird right-click on your Inbox, choose Properties and select the Flags tab. Does it have any flags other than Inbox?

Just Inbox. Are there any other logs generated by TB (except for the activity manager) which could tell me what initiated the delete?

There is this: http://kb.mozillazine.org/Session_logging_for_mail/news