Within Pmail you can create additional subfolders for existing folders of a mailbox. And once these "folders in folders" have been created in Pmail, they are also visible and usable without problems via IMAP in Thunderbird when you connect to this mailbox.
But unfortunately the direct creation of these subfolders (folders in folders) in Thunderbird is not possible, although in Thunderbird settings "IMAP Server supports Folders in Folders" is checked.

Has anybody a hint in this regard?
Does Mercury 4.91 better supports "folder in folder"?

Not sure how Thunderbird handles this, but try creating a session log and see if there are any obvious errors.

Hi Rolf,
Many thanks for your reply. After further investigation and asking other Thunderbird forums the following has been turned out:

When you're going to create a new "main" folder in Thunderbird which is connected to your Mercury IMAP mailbox, you have to name it "folder/", means you give it a name followed by a slash! Then Thunderbird creates a special folder (marked in italic letters) where you are able to create subfolders within.

Ah, good to know! I suppose email clients can have more or less obvious ways of handling such settings.

Hello Joerg - this problem has caused me many headache in the past with TBird...I would only backup your suggestion - I now use Pegasus as a 'master' copy of the folder structure....make all the changes there & let them percolate out in iMAp.

Happy New Year Guys,

Hi Invictus,
Yes, we're using Pmail also since decades. But while Mercury is still running fine and mostly without any problems, Pmail becomes more and more "old fashion" and complicated to administrate in a business environment with many users. Nevertheless the creating of user's folder structure or automatic mail replies will still set up by Pmail.
But I try to solve any problems more and more by Mercury and TB only.

Spot on - Pegasus is a great versatile client ....but when we get younger people into the work environment they are a bit 'man, this is so nineties!' & go down the RCube/Tbird route. Vers 5....bring it on!!