Email from my domain

Hello all.

I just recently transferred my domain name to Register.com and I'm now having problems setting up and managing email accounts. Can someone walk me through what needs to be done on the server side and on the domain host (mx records) side.

I run my own IIS server.
I have an SMTP server running.
Register.com is my domain host and I use there DNS.
I have A records set up to point to my IP, then use host headers on my IIS machine to handle my subdomains.

I would like to be able to send email and receive email using webmaster@mydomain.com or contact@mydomain.com etc.

Thanks for the help,
Ryan