Talk:Thunderbird:Help Documentation:The Options Window

hidden pref: delete mail from server when moved from inbox

This was in 1.5.x and removed from the GUI in 2.0.x

"This adds a hidden pref, "mail.pop3.deleteFromServerOnMove", which defaults to false. If turned on with the config editor, you'll get the 1.5 behavior of deleting a message from the server when moving the message anywhere. The new default behavior is to only delete from server when the message is deleted/moved to the trash." via David Bienvenu via [1]