Apple Mail send problems
I use Apple Mail with Big Sur 11.6.5, and have accounts for 2 Exchange mailboxes and my iCloud (@mac) email. Yesterday, any attempt to send mail from any of the 3 accounts through Apple Mail failed. The messages instead all go into Outbox - On My Mac and never send.
I can use the Exchange accounts on their web interfaces normally, and my iCloud send works from the web interface.
I have tried deleting and then adding the iCloud mailbox to Apple Mail, but to no avail.
Have never had this problem before in decades of using Apple Mail.
Any suggestions welcomed with thanks!
MacBook Pro 15″, macOS 10.12