messages left in outbox

my messages are piling up in my outbox, and only leave when I restart my PC - not very convenient! I have tried opening them and sending them again, but thus far they won’t go! When I hit send and receive, I get “an error occurred” but no further explanation.

in the first place please open the account settings and use the Diagnose/Fix button under the tab Diagnostics. Then inform me of the result.