Error 550 ?

[Psabs�]

I'm have one server with qmail !  the domain is test.com  and host is
serv1 ! qmail internal is very good ,  but when I send one message for
out of the net (for one user in internet ) , he returns error 550 "
don't is possible deliver message for us " !!   what is this ??  is DNS
?? this server is under of firewall with NAT !!  

Thanks for any help !!  And sorry my bad english :)