All SMTP servers are showing "Offline"

The Mail App on my MacBook running Monterey has suddenly stopped sending mail.

When I check the SMTP server's status they ALL show 'Offline' - Mail, Hotmail, etc.

I have not changed any of the Mail or the server settings - incoming mail via the IMAP servers are all fine.

There have been no changes on the router - iPhone and iPad Mail is working normally via Wi-Fi.

Any ideas?