I plan to establish small esp company something like mailchimp there is person told me that there is something called postmaster that i have to get if i want to become bulk sender with millions of sending volume and all go to inbox as long my customers and my servers follow rules what i get from him that all esp companies and their postmasters (yahoo -gmail - aol - etc) follow this postmaster i did not reach to anything offical in my searching about this postmaster till now i will really appreciate if anybody here have any info on what is this thing and how to get this support and license or offical link for this postmaster
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