If the error is "Sorry,_I_couldn't_find_any_hos(ts by that name)" - then the mailserver can't look up the domain. (assuming the address you're trying to send to has, in fact, a valid domain. Check for typos).
- Make sure you didn't firewall the qmail machine off to the point where it's not allowed to open a port and do dns lookups/get answers.
- Make sure the qmail machine has valid nameservers to use.
Please provide the full error next time when you ask a question, and you'll probably get an answer much faster.