Hi.

Check out PHPMailer.
(See an example of how to use it here)

And there is always the old mail function, if you need something simpler.

If you are asking for a whole admin setup your opening up a can of worms that is a very time consuming and complex task else check out Atlis link to PHPMailer to send mail via php.

OmniX has a point if you do not have this knowledge yourself as of now, then you are looking at spending months trying to build a system like this (with ALLOT of help from other developers), or just hire a freelancer and have it done in a week (if not less). The only disadvantage of the second option is that you will not be learning anything.