apple mail stopped sending

weird Apple mail problem

2 of my accounts suddenly stopped sending mail. No error message, no "cannot contact server' message. no password error, no nothing! Just does nothing apart from put message in outbox.

I've restarted, zapped pram, and repaired preferences. I have email working fine on my other 2 Apple Mail accounts. The mail servers are definitely up and I definitely have email connection

The issue happened out of the blue - no changes to accounts or anything

any ideas? Not sure if this is in wrong group but couldn't find a group for Mail