Does Postbox have files named msgFilterRules.dat in its profile, i.e. in the folders that contain the data for each account? In TB such folders are e.g. ImapMail/imap.gmail.com or Mail/pop.mail.yahoo.com.

Thanks a lot. Yes there are files named msgFilterRules.dat in the various folders corresponding to the various accounts. I copied them in the relevant folders of TB. The list of filter names appear in TB but when I try to open a filter I get an alert saying that the program cannot understand it.

The .dat files can be opened in a text editor, and the ones in TB that don't have any rules have this:

version="9" logging="no"

so I suggest you look at the ones in Postbox to see if they have something different that might make them incompatible.

Thanks a lot. The .dat fils in Postbox (and consequently those I copied to TB) all have version="9" logging="no" Should I replace the above by something else in TB?

No, if the Postbox 'header' is the same as the one in TB, I can't say why the Postbox files that replace the TB files aren't viewable in TB in Tools/Message Filters. Check that when you copied the Postbox files to the TB profile, overwriting the existing files, TB was closed.

Yes, it was closed.

I found a solution!!

1. Close both PostBox and Thunderbird.
2. Create a copy of the PostBox version of the file msgFilterRules.dat.
3. Edit the copy. Remove every line that contains global="no" including the paragraph line it is on. In other words, don't leave a blank lie.
4. Save the changes
5. You can then copy the new file to the Thunderbird directory location overwriting the old one. Or you can open the Thunderbird version with a text editor, open the Postbox version with a text editor, and copy and paste the entire contents of the PostBox version into the Thunderbird version file.

On my Windows 10, the files were located in the following directories. Hope this helps you find yours:

• PostBox: %APPDATA%\Roaming\PostboxApp\Profiles\34yz6j6e.default\ImapMailFilters\[myserver]\msgFilterRules.dat
• Thunderbird: %APPDATA%\Roaming\thunderbird\Profiles\a7f47ovg.default-release\ImapMail\[myserver]\

You can then open your Thunderbird and it should be working now.

Update: you may find that the folders component did not import. If that's the case, it's likely there is a case difference. Yes, it turns out that the program wants the path for the folder to have the same case.

In my case, it turned out that I had Xxxx@gmail.com in Postbox, but I had xxxx@gmail.com in Thunderbird. All I had to do was a find and replace to fix the case and the problem was resolved.