email bouncing back

We have a mailmax email server. Some emails are bouncing back. This is completely random. Sometimes it will go and sometimes it wont to the same person/domain.

Here is the header:

Subject: Undeliverable Mail Returned to Sender
>To: <XXXXXXX@XXXX.com>
>
>*** This message was automatically generated by the MailMax Error
>Responder ***
>
>Sorry, your message from <XXXXX@XXXXX.com> to
><XXXXX@XXXX.com> could not be delivered. The specific error is:
>
>   Cannot resolve the IP address of the following domain: XXXXXX.com
>
>
>This is permanent error, and the message will not be retried any further.
>
>=== The Original Message Follows ===
>
>Received: from [67.155.XXXX] by XXXXXX.com [67.155.XXXX] with
>SmartMax MailMax for Audrey.Williams@selective.com; Fri, 25 Jul 2008
>12:14:53 -0400
>Return-Path: <XXXX@XXXXX.com>
>X-SmartMax-AuthUser:
>Message-Id: <5.1.0.14.2.20080725121224.03618e30@XXXXXXX.com>
>X-Sender: XXXXXXXX@mail.XXXXXXXXX.com
>X-Mailer: QUALCOMM Windows Eudora Version 5.1
>Date: Fri, 25 Jul 2008 12:18:25 -0400
>To: "XXXXXXXXXXXX" <XXXXXXXXXXXX>,
>  "XXXXXXXXX" <XXXXXXXXXXX>
>From: XXXXXXXXXXXXXXXX<XXXXXXXXXXXXXXXXXXXXX.com>
>Subject: RE: Invoice Question
>In-Reply-To:
><4C7612F67C63524CBE92018A649A09EE017889F1@XXXXXXX.com>
>References: <5.1.0.14.2.20080715120513.036f8d50@XXXXXXXXXXXXXX.com>
>  <4C7612F67C63524CBE92018A649A09EEE8FCFF@XXXXXXXXXXXXXXX.com>
>  <5.1.0.14.2.20080715120513.036f8d50@XXXXXXXXXXXXXXX.com>
>Mime-Version: 1.0
>Content-Type: multipart/alternative;
>         boundary="=====================_1370037984==_.ALT"

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