When I experimented with the Undelete and Detach features in Thunderbird
1.5 I saw an "Undelete" feature, but I haven't figured out what it is
supposed to do.

If I delete an attachment I get a message saying that it cannot be
undone and then a duplicate of the note is created, without the
attachment. Great! Then if I right click on the original note I get an
option to "undelete message". If I choose that option the note is
deleted! It doesn't even go to Trash. It is actually deleted. This is
using an IMAP server.

The undelete option is only offered on the original note, and only if
I've said to delete an attachment. Don't know what it is actually
supposed to do, but I'm sure this isn't it.