sender domain must resolve

[Claudinei Luis Bianchini]

Hi,
some times I am receiving the message below when I try to deliver emails in
(only) this server. Can I make something to decide this or I only depend on
them?  
I am using qmail+djbdns.

@4000000039a137f52616add4 delivery 65277: deferral: 
Connected_to_200.202.2.1_but_sender_was_rejected./Remote_host_said:
_451_<[EMAIL PROTECTED]>..._Sender_domain_must_resolve/

-- 
Claudinei Luis Bianchini
[EMAIL PROTECTED]