Thanks! 1) I think the fact of paying is already a good enough barrier. The price could be just a couple of dollars, which would be enough to convice sender to

Thanks! 1) I think the fact of paying is already a good enough barrier. The price could be just a couple of dollars, which would be enough to convice sender to spend sometime on writing a personal email instead of using cold mail template. 2) Because of this barrier user will receive much less email + with much relevant content. As a result sender will have much higher chances for reply.