How do hosters do multiple email domains?

[sled_guy]

Once in a while I get myself in trouble by thinking. Tonight is such a case. I am curious if anyone can tell me how hosting companies configure their email servers so that they can host multiple email domains and still have the headers formed correctly.

For example:

I go to a hosting company and I register to host my domain there... domainA.com. I get a web hosting and 5 free pop email accounts. I create an email account called info@domainA.com. When I send email from that account it shows up on the receiving end with the header containing all information as if it came from a server owned and operated by domainA.com.

I then register another domain, domainB.com at the same hosting company. Again setting up an email account called info@domainB.com. When I send email from that address the header on the receiving end now has all the appropriate information in it that says it come from domainB.com.

How do they do that. If it isn't right then a lot of the spam catch tools, especially at AOL, Yahoo and MSN catch it as suspected spam.

I'm just curious.

Dan

[farslayer]

Through the use of virtual domains.

Heres a pair of how-to's you could use to setup a server for hosting web abd email for multiple domains..
http://www.howtoforge.com/perfect_setup_debian_sarge
http://www.howtoforge.com/virtual_po..._quota_courier

Here is an overview of how virtual domains function, both IP basded and name based..
http://netwinsite.com/surgemail/help/domains.htm