You have to meet the following conditions:
1) Have a static IP at your home/office to host your mail server. It is required by some RFCs (request for comments) and almost all ISP mail servers automatically reject incoming mail from non-static mail servers.
2) Point your DNS records as (try to avoid CNAME records):
www A x.x.x.x (your webhost IP)
mail A y.y.y.y (your home/office IP)
mail MX mail.yourdomain.com
3) Ask your home/office ISP to provide a PTR record to your IP (for the same reasons stated in 1).
3) Use a SPF record (TXT) to protect your mail server. See this website for instructions/wizard:
http://www.openspf.org/Nuno