Thanks again Sharhari,
As I wrote, my wife can use her mac.com account, so there is nothing outside my computer which has changed. It has to be something in my clean build of Catalina or other software I may have installed. I'm just looking for a hint as to what that might be or how to find it. I have just checked my System Preferences and the firewall is off, and anyway, the problem is outgoing email which shouldn't be affected by the firewall. Incoming email works fine.
I can send Mail messages from 2 other email addresses.
Something is creating the wrong format for mail messages from the mac.com account.
The response when I try to send mail out from my mac.com account is, with my IP address changed to x:
Cannot send message using the server iCloud
Verify that you have addressed this message correctly. Check your SMTP server settings in Mail preferences and verify any advanced settings with your system administrator.
The server response was: <hostxx-xxx-xxx-xx.range81-151.btcentralplus.com[xx.xxx.xxx.xx]>: Client host rejected: Access denied
In order to see the raw source, I wrote a message to myself with Subject Test and no content and tried to send which failed, so I was able to look at its raw source which is below with my email replaced by xxx:
Subject: Test
Mime-Version: 1.0 (Mac OS X Mail 13.4 \(3608.80.23.2.2\))
Content-Type: text/html;
charset=us-ascii
X-Apple-Base-Url: x-msg://22/
X-Universally-Unique-Identifier: D2397254-E0B2-4F45-BBF1-1ABC05C55F20
X-Apple-Mail-Remote-Attachments: YES
From: xxx <xxx@mac.com>
X-Apple-Windows-Friendly: 1
Date: Mon, 22 Jun 2020 08:45:24 +0100
X-Apple-Mail-Signature: SKIP_SIGNATURE
Content-Transfer-Encoding: 7bit
Message-Id: <ACC80E47-3728-46D4-B992-0CA36F25DC88@mac.com>
X-Uniform-Type-Identifier: com.apple.mail-draft
To: xxx <xxx@mac.com>
<html><head></head><body dir="auto" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div class="ApplePlainTextBody"></div></body></html>
