I can't send new emails, but replying and forwarding works.

I can no longer send new emails via Apple Mail from my 2017 iMac. I can reply to and forward emails and they are sent, but the newly created emails just disappear, they don't arrive and are nowhere to be seen in my Apple Mail app, not in Sent, not in Draft.

Can anyone help?

Software is Ventura, 13.7.2