Are you sure you're dragging from an address book to a mailing list, and not to another address book? The way it normally works, and works on my setup, is that when a contact is added to a mailing list by drag and drop, it's added to the list, but remains in the address book. If a contact is dragged from one address book to another, the contact is moved, i.e. it is removed from the source and added to the target. Holding Ctrl while dragging and dropping will copy the contact, not move it.

Nope. Doing it as you say, dragging with a left click, places it in the mailing list AND removes it from the main personal address book. I am not holding down any key and am using the left mouse button.

Are you doing the drag and drop from the address book contact list to the mailing list icon, not the opened window of the mailing list? Try creating a new test address book, add a test contact manually, create a test mailing list in the book, then add the contact by drag and drop (onto the list icon).

I tried it again with a new book "Test book" and a false name in the personal address book and the drag-n-drop still REMOVED the entry rather than copying it as it should.

One thought. All the drag-n-drop routines are actually part of the Win10 OS. Tbird is simply calling that routine -- correct? The failure may likely be in the OS.

I have the program CCleaner. I wonder if having it check the registry could be helpful? I used this many times without any problems. Don

It's working properly here with TB 102/W10/11, as it always has. I don't think it's affected by the underlying OS. Remember, when you create the mailing list, name it and close it, then drop contacts on the list icon in the left pane.

CCleaner is well-known to cause problems if it is allowed to act on TB, although I haven't heard of it being related to the issue of adding contacts to lists.