After my recent update to Sequoia 15.3 my outgoing Mail stopped working. I found the cause and fix.

After my recent update to Sequoia 15.3,

When trying to send out any mail documents from my Mac Studio to iCloud and Exchange they failed to send, with the returned message

(My iPad and phone were not affected.)

It turned out that somehow my outgoing iCloud and Exchange Server settings under Sequoia had been changed to iCloud (Offline)

When changed to the choice of iCloud, all Mail functions returned to normal.

Exchange needed a similar change.