First things first see if your internet provider blocks port 25 also ask them if running a mail server would violate their terms of service. I wouldnt want to lead you down the wrong road! Also if you can ask them for an upgrade in service they can and will provide a static IP address for a small monthly fee. Then you can set the box up normally.
If you dont want to spend the money or your company told you no then you can look into a web service called dynDns located at www.dyndns.org .
Basically it has a client that goes on your box and when it changes IP addresses ( mine has not changed in the last 3 years on Cablevision) it updates it on the web. So you will have a servername like Yourname.DynDns.Org that will always pojnt to your box regardless of what internet IP it has.
I know dyndns.orf i have created my domain account on it tell me one thing what to write in MX Hostname will it be like mycompany.com or the domain name i have created on dyndns
First of all if your on the internet you hava a public IP address, just not static!
Second, I guarantee you’ll have problems sending e-mail thru exchange on a dynamic IP address, many ISP’s are now blocking dynamic IP ranges that use a mail server.
Third, as far as receiving e-mail, you must have a registered domain name from a registrar like doster, godaddy, etc., then use dyndns or zoneedit(both free) to point your domain name to your dynamic IP. I use directupdate to monitor my IP and change it on zoneedit as needed.