To save/append a message to an mbox file currently requires the folder containing the mbox file to be added as a mailbox to Claws, and the actual mbox needs to be created (even though it exists!). The new mbox will become a first class Claws mailbox, scanned for new messages, and so on. Often is it desired to just save a message to an external mbox. I'd like to request an enhancement so that this will be possible with (e.g.) a simple right-click > save/append mbox.