I’m not saying that, simply that you should try it and see if it resolves the problem, first try it with a subject line and then with someone in the To field and see what happens.

It really shouldn’t cause you friend much additional effort to put a subject in the email or to send it with a real email address in the To field.

This surely would be better than switching off the heuristics for all email to get around a problem with email from one source.

Its a case of security over a little extra effort, convenience, or switch off heuristics or doing nothing and put up with the warnings. The decision is entirely yours.