Mr Red wrote: ... there has to be something in the "To" field and that can be your own address.

Your email client or webmail may require it but email in general doesn't, and Thunderbird doesn't. Some email clients put 'Undisclosed recipient' in the To field of emails received (not sent) that have only BCC.

I've now installed the 'Use BCC instead' addon and set it to warn me if there are more than 3 non-BCC recipients. Might save me some embarrassment.