Hey, I never thought about this before, but in general for people sending stuff to family in the S.O maybe they could number their messages in such a way that the family member would be able to tell that this was the 5th or 6th message sent etc. That way if any messages were never relayed due to 'entheta', the next entheta-free message the family member received would alert the family member to the fact their mail has been being withheld from them.
I think that is a super way to do it Minnie. Tampering with the mail is a Federal crime in many places. (except witthin scientology organizations which are considered above the law)