For some time I have been playing around on Earn.com, formerly known as 21.co - a service that rewards you for responding to email. It pays in Bitcoin and was bought by Coinbase last year. At first I was a little reluctant; how much spam was I inviting over with this service? But I have been pleasantly surprised by the interface, the service and the messages I got.
From the other side, the service enables others to send emails to specific people. Sp much so, they are willing to pay so that they know that person has actually read their message. There are various categories to browse through, from developpers, startup CEO’s and Bitcoin HODLers.
In general, most of the messages I have received are related to new tokens that are being launched. Either they are looking to promote their channels (mainly Telegram) or check the interest in an upcoming ICO of sorts.
Check it out and create a profile at Earn.com for yourself if you like. Disclosure: the link is my affiliate link.