Problems with qmail and dns

[martin]

Hi,
With some mail servers,when sending mail, I'm getting this problem, but
everything seems well configured for my domain which is: sintesoft.net
(check it out with nslookup)

Any idea?.

o_206.250.5.2_but_sender_was_rejected./Remote_host_said:_451_<[EMAIL PROTECTED]>..._Sender_domain_must_resolve/