Apple Mail won't send particular email

Here's an odd one if anyone is interested. Mail wll not send a particular email but it will send everything else.

The SMTP server is connected.

The email address is phishing@hmrc.gsi.gov.uk. I have copied this address and then after it failed I retyped the address. (It is the official HM Customs and Revenue spam/phishing reporting email address.)

The email sent is a forwarded spam/phishing mail received earlier.

I have tried with three accounts: 2 have the same domain name but the other is different.

All other outgoing mail is OK. A mail requesting HMRC confirm that the address is correct was sent OK!

Incoming mail is not affected.

What's going on?