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I have 2 users getting the following message from the same external email server for 6 recipients:<br />
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This is a delivery status notification, automatically generated by MTA domain1 on Thu, 24 Jan 2013 14:16:18 -0500 Regarding recipient(s) : user1@domain1.com Delivery status : Failed. None of the mail servers for the destination domain have so far responded. (domain1.com).<br />
We have been attempting delivery for 5 times.<br />
Maximum delivery tries attempted. Please contact your administrator to contact the destination domain or resend your message. Delivery failed.<br />
Message Information: <br />
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Sender &nbsp; &nbsp; : user1@domain2.com <br />
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Recipient &nbsp;: user1@domain1.com <br />
MTA Response :510 The original message headers are included as attachment.
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I have 2 users getting the following message from the same external email server for 6 recipients:

This is a delivery status notification, automatically generated by MTA domain1 on Thu, 24 Jan 2013 14:16:18 -0500 Regarding recipient(s) : user1@domain1.com Delivery status : Failed. None of the mail servers for the destination domain have so far responded. (domain1.com).
We have been attempting delivery for 5 times.
Maximum delivery tries attempted. Please contact your administrator to contact the destination domain or resend your message. Delivery failed.
Message Information: 
	Sender     : user1@domain2.com 
	Recipient  : user1@domain1.com 
MTA Response :510 The original message headers are included as attachment.

why am I getting bounce backs from external email server?

Interactions between email servers and clients are governed by email protocols. The three most common email protocols are POP, IMAP and MAPI. Most email software operates under one of these (and many products support more than one).  The correct protocol must be selected, and correctly configured, if you want your email account to work.

Email Protocols

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

An email client, email reader or more formally mail user agent (MUA) is a computer program in the category of groupware environments used to access and manage a user's email. A web application that provides message management, composition, and reception functions may internally act as an email client; as a whole, it is commonly referred to as webmail. Likewise, email client may be referred to a piece of computer hardware or software whose primary or most visible role is to work as an email client. Email clients can also have other systems, like calendars, notes and contact managers.