Sender domain is empty - Sender verify failed

[jamesbond]

Sender verify is failing for a certain domain, and I noticed in the logs it says 'Sender domain is empty'. I haven't been able to figure out what that means exactly. Does anyone have an idea?

007-03-09 10:39:40 H=out.mail.xxxxxxxx.net (xxxx.xxxxxxxx.xx) [xx.xxx.3.120] sender verify fail for <xxxxx.xxxxx@xxxxxx.com>: response to "RCPT TO:<xxxxx.xxxxx@xxxxxx.com>" from mail.xxxxxxx.net [xxx.xx.201.11] was: 550 5.1.1 <xxxxx.xxxxx@xxxxxx.com>... Sender domain is empty
2007-03-09 10:39:40 H=out.mail.xxxxxxxx.net (xxxx.xxxxxxxx.xx) [xx.xxx.3.120] F=<xxxxx.xxxxx@xxxxxx.com> rejected RCPT <xxxxxxxxxx@xxxxxxxxxx.xx>: Sender verify failed

[Zion Ahead]

>>> RCPT TO:<postmaster@localdomain.net>
<<< 550-"The recipient cannot be verified. Please check all recipients of this 550 message to verify they are valid."


This message was created automatically by mail
delivery software.

A message that you sent could not be delivered to
one or more of its
recipients. This is a permanent error. The following
address(es) failed:

user@localdomain.net
SMTP error from remote mailer after RCPT
TO:<user@localdomain.net>:
host localdomain.net [xxx.xxx.65.136]: 550-Callback
setup failed while
verifying <user@earthlink.net>
550-(result of an earlier callout reused).
550-The initial connection, or a HELO or MAIL
FROM:<> command was
550-rejected. Refusing MAIL FROM:<> does not
help fight spam, disregards
550-RFC requirements, and stops you from
receiving standard bounce
550-messages. This host does not accept mail
from domains whose servers
550-refuse bounc

[jamesbond]

Is this a reply to my post?