email project on linux server

good morning i have a domain name - it is on a linux server - what i would like to do is have my members email each other inside of my server using an activation key - user name idea which i will supply to them upon signup? for example my users will be able to see each others profiles and pictures for free prior to paying for their membership fee - i will put their profiles and pictures on the site myself using ftp - but to be able to email each other from inside of my members area they have to pay the monthly subscription service to do that. a members area could be created inside of the linux server which will use their activation key - user names - so their emails will begin to work to each other - this is from inside of my members area which we can create from inside of the linux server - without the activation - user key - my members will only be able to view each others profiles and not be able to email each other - i know how to upload and ftp to my sever etc.. to fine tune etc.. my server is on linux - i have cpanel to administer it with and the ftp login information ## Deliverables the code can be developed by my programmer - this code will enable my members to email each other from within a members area -