Service idea: Spam filter.

Regular mail service running along the following chain:

1. register, get public mail
2. indicate the real e-mail to which to send mail
3. on all sites we publish “public”, issued during the registration mail

Service:
')
1. Sends a letter to all incoming emails - a spam test with the same captcha, for example
2. If the sender has passed the spam test - send to the recipient on the “real” mail
3. Add the sender to the “trusted” list for this user.

Lecture hall:

1. Service in demand. Spam - zadolbal all.

Chips:

1. The service is free.
2. You can score a list of “trusted” domain-recipients manually, import from address books, etc.
3. Move auto-delete spam to the folder, etc. etc.

Monetization:

1. Advertising at the end of the letter) or a subscription to the choice. Personally, I am ready for voluntary advertising in the necessary letters, because I hesitated to waste time on rubbish raking in search of important letters.

UPD: Here's a flash drive that demonstrates the essence of the idea: corp.spamarrest.com/howitworks2

UPD2: Russian free analogue found:
mail.km.ru
Problem solved