I would recommend you setting up a master server (web/db etc.) and a slave server for mail, like here: http://www.howtoforge.com/multiserve...th-ispconfig-3
Sending emails will be possible from the web server as mostly the php mail function is used and if not, the script will likely let you config a smtp server.
I don't get the DNS question? Of course you need A records for both servers and the MX for the domain should point to the mail server.
Or what do you mean?