This happens on multiple machines - two MacBook Pros and one Apple Studio, all using Ventura 13.1. Note that this issue has persisted over the last few MacOSs.

Check the received message and compare the contents with the message you sent. It's possible your email provider added e.g. some sort of advertising to the message, which would cause the "Digital Signature is Not Valid" error.

The received message is the same as the sent message except in the MIME headers:

Saved sent message:

Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: 8bit

and the BCC received message is:

Content-Type: text/html; charset="utf-8" Content-Transfer-Encoding: base64

Could changing from 8bit coding to base64 coding be the problem? I just checked Bugzilla and found this: https://bugzilla.mozilla.org/show_bug.cgi?id=1794768

thanks

Could changing from 8bit coding to base64 coding be the problem?

Yes, I think so. But I'm not sure why the coding changes.

You've updated the Bugzilla entry 1794768. How can I see if the fix is on the roadmap, the status, or anything else helpful?

Just watch the bug. If there is any activity you'll see it there.

As a workaround you can try to set the pref mail.strictly_mime to true. This will enforce 'quoted-printable' content transfer encoding.