ports needed to be open for a normal hosting operation

You may find some info in http://www.ehcp.net/helper/yardim/cat.php?cid=6&qid=59
or helper homepage, http://www.ehcp.net/helper/yardim/cats.php

Basicly,
You need to open ports 20,21,22,25,53,80,110,143 on your server and on your modem/firewall/router...
if you have no firewall on your server, you dont need to do anything on your server... But, some users put firewall or extreme security programs so that nobody can reach them... Please avoid this...

You may check your open ports at http://www.ehcp.net/other/portstatus.php

Comments

The link given by you for testing IP really helped me in detecting whether ports are open or not. Normally people will not concentrate on ports whether they are open or not they think that ports are nothing to do with hosting operation but it is not true when your firewall is on,you have to cross check ports otherwise you will face problem.