This is using the current 1.5.2 snapshot -
The undelete button doesn't always show up when it's
If "trash_folder=none" is in the users prefs, the
undelete button will not show up.
This gets set when one submits/saves Folder
If you remove the "trash_folder=none" from the user's
pref file, everything works as expected.
It looks like the test in functions/mailbox_display.php
needs to updated to also show the undelete button if
trash_folder is set to none.
When viewing a deleted message, it should have an
undelete button to allow you to undelete the message
you are viewing.