If I select multiple mailboxes in the Mailboxes window, either deliberately or by accident (see issue about mailbox drag behaviour), GNUMail immediately raises an error: "You must select a valid mailbox to open!". This happens quite easyily.

It would be better to just display a hint that multiple mailboxes were selected in the messages list.

Or, as a quick fix, the mailboxes list could be made single-selection. But I believe a multiple selections should be possbile eventually, in order to delete or move multiple mailbox folders at once.

GNUMail 1.30 (precompiled) on Mac OS X 10.9

PS: Thank you for GNUMail :)