Hey @timcliff,
That is precisely the idea! You set how much someone must transfer you directly in order to send you a Steemail message. Steemit.com receives nothing. All postage fees go directly to the recipient unless they choose to return it to the sender.
Regarding a 0 SBD postage fee for some users that would be ideal, but I am not sure if there is a minumum Steem tx amount. Either way free Steemails could be accomplished by automating the return postage for certain users.
Thanks for the comments.
Ah, I see. I had to re-read it again. [It's late :)]
It may be a good idea for it to also 'burn' a small amount of SBD as a 'postage fee'. It would basically generate a community benefit from the system's use.
Not a bad idea at all and worth considering if anyone decides to develop the project.