Quoting Koen Martens <spf(_at_)metro(_dot_)cx>:
Putting a special password in the subject
header is peanuts,
it's a 2 second thing. It's something your clients become accustomed too very
fast.
You're putting the cart before the horse. Propose an authentication standard,
THEN get it built into tools so it can be trivially used by non-technical
users, THEN require its adoption.
If users had a simple form into which they could put "permission-to-send
password:" ONCE and have it happen automatically, it would be acceptable. But
it has to be implemented in all major MUAs BEFORE you mandate it.
Don't mandate it and put any burden of accomodation at all on the end users.