Exclamation Mails from "rider.php.va.us.xda-developers.com"

Hi!

Every day my mail server rejects mails from "www@rider.php.va.us.xda-developers.com" and "www@shooter.php.va.us.xda-developers.com" because the domain used as sender has no valid DNS entry.

Maybe there is something misconfigured?!

Code:
Sender address rejected: Domain not found (total: 31)
          21   www@rider.php.va.us.xda-developers.com
          10   www@shooter.php.va.us.xda-developers.com
Code:
Dec  9 08:42:50 mail postfix/smtpd[3196]: NOQUEUE: reject: RCPT from unknown[207.244.73.227]: 450 4.1.8 <www@shooter.php.va.us.xda-developers.com>: Sender address rejected: Domain not found; from=<www@shooter.php.va.us.xda-developers.com> to=<xda@XXXXX.de> proto=ESMTP helo=<shooter.php.va.us.xda-developers.com>

[...]

Dec 10 06:00:56 mail postfix/smtpd[5644]: NOQUEUE: reject: RCPT from unknown[207.244.73.228]: 450 4.1.8 <www@rider.php.va.us.xda-developers.com>: Sender address rejected: Domain not found; from=<www@rider.php.va.us.xda-developers.com> to=<xda@XXXXX.de> proto=ESMTP helo=<rider.php.va.us.xda-developers.com>