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That's what I thought, but is there a way I can prove it? I already contacted their administrator, but he claims that the problem is mine as it is not even reaching their exchange server... 
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That's what I thought, but is there a way I can prove it? I already contacted their administrator, but he claims that the problem is mine as it is not even reaching their exchange server...

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Your message did not reach some or all of the intended recipients.<br />
&nbsp; &nbsp; &nbsp; Subject: &nbsp;FW: Undeliverable e-mail<br />
&nbsp; &nbsp; &nbsp; Sent: &nbsp; &nbsp; 5/23/2008 9:30 AM<br />
The following recipient(s) cannot be reached:<br />
&nbsp; &nbsp; &nbsp; ******@xxxxxxxxx.org on 5/23/2008 9:31 AM<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; There was a SMTP communication problem with the recipient's email server. &nbsp;Please contact your system administrator.<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &lt;server.domain.local #5.5.0 smtp;530 authentication required for relay (#5.7.1)&gt;<br />
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This is the return e-mail I am recieving when attemting to send a e-mail to an individual working for xxxxxxx company. 
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Your message did not reach some or all of the intended recipients.
      Subject:  FW: Undeliverable e-mail
      Sent:     5/23/2008 9:30 AM
The following recipient(s) cannot be reached:
      ******@xxxxxxxxx.org on 5/23/2008 9:31 AM
            There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            <server.domain.local #5.5.0 smtp;530 authentication required for relay (#5.7.1)>


This is the return e-mail I am recieving when attemting to send a e-mail to an individual working for xxxxxxx company. 

Undeliverable e-mail to one domain only. Getting the following error message:

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

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.

Email Servers

Windows Server 2003 was based on Windows XP and was released in four editions: Web, Standard, Enterprise and Datacenter. It also had derivative versions for clusters, storage and Microsoft’s Small Business Server. Important upgrades included integrating Internet Information Services (IIS), improvements to Active Directory (AD) and Group Policy (GP), and the migration to Automated System Recovery (ASR).