It could be any number of reasons. One thing to check is to verify that the email address in incoming pane is the same as the email address in SMTP server pane. This problem is not uncommon when a user has multiple email accounts with the same provider and has the SMTP server settings crossed.

No. It has only happened maybe half a dozen times, with a correspondent to whom I've sent hundreds of emails (and from whom I've received hundreds). It only started a few days ago. It is extremely annoying. So far, the problem has gone away after half an hour or so. I have only the one email account with this provider.

The mail server responded: Abusive Pattern Detected (ZD/78B9EF1).

That message is from your mail providers mail server. Thunderbird is just the delivery mechanism reporting what the provider gave it to explain why sending failed.

You do not name the mail provider that is giving the error, so guessing is all anyone can do. The actual error code does not appear in a Google search, which is unusual. I am sure your provider will be rejecting mail from more than just you, unless it is a hosted domain and you have subscribed for something special, or you are breaching the providers terms of service.

Are you sure the from address domain (the bit after the @) is also operated by the outgoing server you are attempting to use to send mail. ie if your address is Gill@brown.name, then the SMTP server being used should be one registered in DNS as supporting Brown.name so sending from say Spectrum's SMTP server will cause issues.

The only thing I can find that is similar is with Microsoft/Outlook/Office365 See https://learn.microsoft.com/en-us/defender-office-365/outbound-spam-protection-about