Mail can’t send message using iCloud SMTP server (macOS Ventura): “message content rejected”

Before posting this question, I searched for: “The server response was: message content rejected” and got (0) results.

From the screenshot attached above you can see that:

• the message is being sent from an @icloud.com account;
• the correct SMTP server is being used;
• Connection Doctor shows no issues with the iCloud SMTP server.

Problems with the macOS Mail app started (3) days ago after installing the macOS 13.7.5 Security Update. Mail repeatedly dropped connections to my non-Apple accounts. That took care of itself in 24 hours.

Yesterday, Mail kicked up this same SMTP objection for a similar message containing the raw source code of spam being sent from the same iCloud account to the same addressee. The “message content rejected” was a mystery: I have sent hundreds of emails with raw source code of spam from all kinds of offenders for analysis and reporting to the same service without once having a problem. I had copied the subject line from the raw source content and wondered if that might be causing a problem; I deleted the subject line and keyboarded it. It was delivered.

I had the same problem with a message earlier today, same sender to same addressee containing similar raw source and was able to fix it by keyboarding the subject line again. But this time, no joy.

Everything was fine until the macOS 13.7.5 Security Update.

Before I delete and re-create my iCloud email account, can anyone tell me why Mail server settings spontaneously become corrupt? Why is Mail so fragile and unstable? Every few months, some server connection just fails. Over 30 years I’ve deleted and re-created countless Mail accounts and performed dozens of system upgrades/re-installs trying to permanently resolve these issues. It’s the definition of insanity,

[Edited by Moderator]