SMTP host server

My company is having me research restructuring our email system so our incoming mail from the internet will come to a SMTP server then be routed to an Exchange MTA server then to a mail server containing the users various mailboxes. Right now, SMTP,MTA, and all user mailboxes reside on one server. I am trying to find out what Microsoft recommends on setting up a configuration like this especially the SMTP server. I looked on their site, but couldn’t find the information I need. Can someone point me in the right direction?