• Online Money World
• Earn money (safely)
• PTR (Paid To Read mails)

These sites are often different from each other, but they still exist two variants.

Sites that pay for clicks or actions

For these type of paid mails sites, you will be paid by simply clicking on the links or offers received by mail.
Generally, if it's just simple clicks, you will not earn much.
But if it's actions and clicks, as on the Qassa (formerly Money Millionaire) site, you can easily earn money.

Sites that pay for research and/or registrations

If the webmaster of the concerned paid mails site is serious and pays his members, you can earn at least ten euros or dollars per month.

Since campaigns paid for action (or lead) are better paid by advertising networks and affiliate platforms, the earnings will be higher than on other paid mail sites.

Trusted PTRs (which ask for specific actions) allow you to earn at least $ 10 or € 10 per month, but the amount varies from a site to another site and depends on the seriousness of the webmaster and its members.