iCloud Mail - Intermittent Problems From M2 MBA With TLS Authentication

For some reason Mail on my M2 MBA running Sonoma (14.2.1) sometimes insists on using Port 25 (obsolete) for TLS authentication with the iCloud SMTP server which always fails. Here's a sample log file entry:

READ Jan 27 14:45:01.209 [kCFStreamSocketSecurityLevelTLSv1_2] -- host:p56-smtp.mail.me.com 
-- port:25 -- socket:0x600007b54540 -- thread:0x60000445db00
535 5.7.0 authentication failed

If I enable a VPN, Mail starts using the correct port (587) and continues to do so even after the VPN is disabled:

READ Jan 27 14:56:12.185 [kCFStreamSocketSecurityLevelTLSv1_2] -- host:p56-smtp.mail.me.com 
-- port:587 -- socket:0x600007bd33c0 -- thread:0x600004556e40
235 2.7.0 Authentication successful

I looked at the Server Settings and disabled "Automatically manage connection settings" and see that Mail should be using port 587. But it doesn't with any consistency.

This problem doesn't happen with any other device: Intel iMac, M1 MBA, iPhone, iPad.

Can someone explain to me why Mail on the M2 MBA sometimes uses port 25 for TLS authentication to the iCloud SMTP server? Since automatic mail configuration works on every other device it seems that it ought to work on the M2 MBA.

I've raised this issue with Apple support, but they remain stumped.