cannot send email from new MacBook via iCloud server

Am migrating from an old to a new MacBook Pro (15 inch, 2018, 1Mojave 10.14). When I try to send from iCloud (using Mail on my new Mac) I get this message:

"The server response was: Authentication required prior to MAIL/SAML/SEND/SOML."

Note that:

- I can still RECEIVE messages to my iCloud account in Mail.

- I can still SEND messages via iCloud if I use the web browser interface or my iPhone -- just can't send using Mail on the Macbook.

- I've tried looking up the server settings under Accounts, and everything appears in order (it reports my iCloud account is online and shows the outgoing mail account is iCloud; I can find no place to enter or check additional configuration details).

- I've tried the Mail Connection Doctor and it reports everything is OK with the connection.

- Using Mail on the new MacBook, everything is fine with my existing Gmail and Exchange accounts.

- So the problem is very specifically limited to some subtle configuration related to Mail and iCloud on my new MacBook. But I can't see anything to change?? Please help!

-David