When you say the .dat file is copied into the 'profile folder', are you copying it into the Mail/<popservername> or ImapMail/<imapservername> subfolder, overwriting the existing .dat file? Each account has its own subfolder and .dat file. Is TB closed when you copy in the file?

I copied the .dat file into the folder I specify in account settings -> server settings -> local directory, which is _not_ the default. I did this while Thunderbird was closed, and I have a separate local directory, and thus separate msgFilterRules.dat file, for each account.

Bobz said

I copied the .dat file into the folder I specify in account settings -> server settings -> local directory, which is _not_ the default. I did this while Thunderbird was closed, and I have a separate local directory, and thus separate msgFilterRules.dat file, for each account.

I think you need to provide more information. Where is this "non standard" file located. Local machine or somewhere else for instance.

For a long time support folks have counseled about keeping a profile folder together, if for no other reason that manually specifying random locations for account mail makes for difficult migrations. So you will need to be specific about where exactly things are stored, on what sort of devices etc.

Can you reproduce this using safe mode of your operating system?

• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

In Thunderbird list of accounts, click on account name In "accounts", click on "view settings for this account" In "server settings", under "message storage", set the value for "local storage" The directory I specify is on a local drive.

I have also tried setting filters for this Yahoo account using the dialog box that comes up when I press "Manage message filters". No matter which way (dialog box or copying msgFilterRules.dat file) for this specific account, the filters work for a while, then disappear.

I have no problems using Message filters in 10 other accounts. Each of those accounts has its own local directory with its own msgFilterRules.dat file on the same local drive. So I really don't understand the relevance of safe mode.

I'm running Thunderbird under Windows 10 in a VMWare virtual machine with 3 processors and 5 GB RAM. The virtual drive I use for the local directories has 10 GB free out of 40 GB. Windows Resource Monitor shows none of the resources is near its limit.

The host computer is a Lenovo ThinkPad X1 running Windows 10 with 16 GB RAM and 6 processors. Windows Resource Monitor shows none of its resources is near its limit.

I _think_ I solved the problem.

I got the idea that maybe the problematic (Yahoo) account was accidentally sharing its local folder with another account. Or perhaps two folder names were close enough to confuse Thunderbird. I confirmed that all the folder names are distinct, and the second hypothesis is weird. But at least I had something to try. So I a) shut down Thunderbird b) created a new folder with a name very different from the others c) started Thunderbird d) changed the local folder for the problematic Yahoo account to the new folder e) restarted Thunderbird f) created new filter rules for the problematic Yahoo account

That was almost 12 hours ago, and the rules are still around. I restarted Thunderbird once, as an extra test. I have no good theory to explain why my "method" worked, and I'm generally cautious. So I'll wait a day or two before I mark this is solved. But I'm hopeful.

Message filters are still intact 3 days later, despite at least one accidental shut down. I would feel much more confident if I understood why my "fix" worked. But I'm confident to mark this fixed.