Change the Mail aliases (address)

[Chip_-_Express at cup.portal.com]

I am trying to configure my mail system.  I would like to be able to mail
a letter to the address "name at portal.com" but I can't.  It seems that I
do not have the correct aliases.  I am looking for a manual that describes
the aliases sintax with some examples.  If somebody have the answer to my
question or somebody knows about a good manual I'll be glad to know about
it.
Thank you all and have a nice new year.
My address is udi at luna.portal.com or chip_express at cup.portal.com
In order to send to that previous address today I type portal!portal.com!name
I would like to know the apporpriate alias.