Naveen Agnihotri replied with a fairly detailed explanation to your question... didn't that work? Did you try it? He says that it's all explained in the qmail FAQ and he even posted links to the relative sections.
AFAIK, you have your machine to the relay list. Remember qmail is setup to be secure by default unlike most versions of Sendmail out there.
Take a look at his answer and if it doesn't work let us know.
