Mail on MacBook Air cannot send emails due to authentication errors

MacBook Air M5, macOS Tahoe, Mail 16.0.

Hello,

I have a problem.

Apple Mail receives fine over IMAP, but it cannot send from my own-domain address. Every SMTP attempt fails with:

• Unable to verify account name or password
• or “The sender address was rejected by the server … Authentication required”

What I’ve tested:

• Two different providers. It started with my old host (Plesk/Postfix), and it’s identical now with Infomaniak (mail.infomaniak.com).
• Ports and settings. I tried ports 465 (SSL/TLS) and 587 (STARTTLS) with “Manage connection settings automatically” off, authentication “Password”, and username = full email address.
• The same credentials work from Terminal on this Mac. curl smtps://mail.infomaniak.com:465 -u … sends successfully (exit code 0), and IMAP via curl works too.
• Thunderbird on the same Mac sends fine with the same server.
• Things I ruled out: I recreated the account, removed old trusted certificates from Keychain, uninstalled Malwarebytes and restarted, and checked my firewall (Radio Silence), which doesn’t block Mail.
• With the old server, the Mail connection log showed that the TLS session to the SMTP server never started: port 465 hung, and port 25 fell back to plaintext. Connection Doctor reported SMTP as “no authentication required”.

Has anyone seen Mail fail SMTP on every server while curl and Thunderbird work from the same Mac? Is there a Mail cache, plist or Keychain item I should reset?

Thanks!