Help finding an installing a reliable and freem linux mailserver

I'm running redhat 7.5 -- all my windows mailservers are not stable -- I need someone to help me choose a free linux mailserver and assist me in setting it up ## Deliverables 1) Complete and fully-functional working program(s) in executable form as well as complete source code of all work done. 2) Installation package that will install the software (in ready-to-run condition) on the platform(s) specified in this bid request. 3) Complete ownership and distribution copyrights to all work purchased. ## Platform rh 7.5