Hello,

Yes. That would be great.

Testing for recipient-is-valid includes the check for a valid address or a valid user-ID ...

It should do both. First check for a valid user id. Then check that that user has a valid email address. And at last, as you said, check that that user accepts email.

Testing for sender-is-valid includes the check for a valid address and the validity of the ID of the current user.

I agree.

Best regards
Daniel