iCloud Can't Send Mail from iCloud Account in Apple Mail App

[mac-kitty]

This afternoon I started having problems sending mail from my iCloud account in the Apple Mail app. The error message is "The server response was: <unknown[192.228.49.117]>: Client host rejected: Access denied." Is anyone else having this problem? Thanks.

 

[mac-kitty]

I checked my iMac, and it can send email from iCloud. However, I am still unable to send email from iCloud on my MacBook Air. Both Macs have the same mail settings and are running macOS 15.3.2.