Unable to send emails with 'Hide My Email' after macOS 26.1 update on iMac M1

prior to macOS26 Tahoe, on my iMac M1, macOS26.1, in Mail.app I was able to send mail message to new contact using a "Hide M Email" creating a random email.

Since update to mac0S 26.1, I I am unable to 'send' after creating a new random email in the Mail.app::

The error message is Apple (out) Server is not configured to send using the hide my new random hide my email..... it defaults to my AppleID email ( @mac.com which I never use to send messages, instead all my mail goes through my @iCloud).. I am asked to select a Server ... there is usually two Apple options (one says it's offline).

Appreciate advice about this 'set up' issue, please Garry, Super Senior, non-techo

[Re-Titled by Moderator]

Original Title: Unable to send