[NTLUG:Discuss] mail server help

[m m]

Hi All:
I need help on set virtual host on mail server.
mail server is localhost, with domain abc.com (just an example) 
there are 2 virtual domains: def.com, xyz.com
I like to have my mail send out with form address showing web at xyz.com and support at def.com
can any one help me out how to config correctly on either sendmail or postfix?
any example files are really appreciated!
I have tried googling the answer, and get varies version and no work!
Please help, Thanks.