Outbox/Sent Items?

[wza]

It's always empty. It hasn't stored a single message. Is that intentional?

[MonkeyIsland]

By default yes.

If you want your message to be saved there, you need to tick "Save a copy in my outbox" below the message box.

If you want it to be always on, you can visit this url:
https://www.tus-wa.com/pm/settings/

Then tick "Save a copy of each personal message in my sent items by default."