@fellowtraveller (the OP):

I'm in a group which is about 40% people over 50 who don't use the internet. Trying to get them to use Facebook is a chore, in and of itself. When I notified them via Facebook that there was an event coming up, all but one of them said they never got the message.

They rather enjoy being able to keep in touch with one another via email. However, I'm also wondering about the potential for list-service emailing which would allow for multiple recipients based on an opt-in (a proof-of-work opt-in mechanism which might allow them to join a list of recipients who have all agreed using a similar mechanism)?

This would have to be built into the protocol itself, as there are around 2000 members in this group who are in need of listserv-style emailing functions, and when we update they are unwilling to go to a web site (as I said, they barely conceive of email, and I would have to sell them on the idea that this is similar, perhaps even to the point that they would absolutely reject it, but for the simplicity of their use, etc.).

There are reasons for mass emails which aren't actually spam. Church groups, civic clubs, political parties, internal messaging, etc., all benefit from mass emails.

However, I like the idea of preventing spam by requiring a proof-of-work for someone you don't actually know, especially if they don't really want your contact. But what I'm wondering is this: might I suggest that if a pre-existing key exchange has occurred (PGP, Bithash, scrypt, etc.) that there be an opt-in standing agreement which can be revoked by the recipient at any time?

Just an idea. Hope it's not too complex to implement. Not being a programmer, I come up with a lot of those.