$5 is probably too much, tho. I'd be looking more at the $.2 to $1 range.
Maybe a 3 to 4 tier inbox. Known and trusted user being able to contact you without paying, a high value inbox for the $1+ range, a low value inbox for the $.2 range emails wont be auto-deleted in and a very low value inbox emails will be deleted in depending on the amount paid, with free mails being gone within e.g. an hour, all the way up to e.g. a month for $.19 mails.
Then unify those inboxes and set up notifications to the users' likings.
Also, I'd normalize e.g. 10% going to the e-mail service providers and enshrine that amount into the protocol right away. Otherwise the protocol wont get a lot of attention from the major providers and if it does, the provider taking his share is going to become normalized anyway. But then the split isn't going to be in favor for the users. Which isn't negative per-se, but it'd be nice to have at least one type of service where this is split is reversed. And it is fair to assume whoever takes the larger split has more influence on the prices, potentially either making this feature useless or pricing very casual users out of the service.
Maybe a 3 to 4 tier inbox. Known and trusted user being able to contact you without paying, a high value inbox for the $1+ range, a low value inbox for the $.2 range emails wont be auto-deleted in and a very low value inbox emails will be deleted in depending on the amount paid, with free mails being gone within e.g. an hour, all the way up to e.g. a month for $.19 mails.
Then unify those inboxes and set up notifications to the users' likings.
Also, I'd normalize e.g. 10% going to the e-mail service providers and enshrine that amount into the protocol right away. Otherwise the protocol wont get a lot of attention from the major providers and if it does, the provider taking his share is going to become normalized anyway. But then the split isn't going to be in favor for the users. Which isn't negative per-se, but it'd be nice to have at least one type of service where this is split is reversed. And it is fair to assume whoever takes the larger split has more influence on the prices, potentially either making this feature useless or pricing very casual users out of the service.