Question : Relaying denied. Proper authentication required

Hello Everyone:

When I email this customer, I get a bounce back with the following response:
Delivery has failed to these recipients or distribution lists:

[email protected]rom.com
Your message wasn't delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator.

The following organization rejected your message: hallsofbeerdom.beeradvocate.com.

  _____  

Sent by Microsoft Exchange Server 2007







Diagnostic information for administrators:

Generating server: MailServer.TitanCo.local

[email protected]rom.com
hallsofbeerdom.beeradvocate.com #550 5.7.1 trom.com>... Relaying denied. Proper authentication required. ##


Does anyone have any idea's how to fix this?  All other emails to other people outside of the organization is working just fine.

Thanks

Answer : Relaying denied. Proper authentication required

It seems the issue is on the remote end, bcoz.
When I did telnet from my end I got the same results.
220 hallsofbeerdom.beeradvocate.com ESMTP Sendmail 8.12.8p1/8.12.8; Wed, 14 Jan
2009 11:06:53 -0800 (PST)
ehlo abc.com
250-hallsofbeerdom.beeradvocate.com Hello [xx.xx.xx.xx], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE
250-DSN
250-ETRN
250-AUTH LOGIN
250-DELIVERBY
250 HELP
mail from:admin@titancontracting.com250 2.1.0 [email protected]... Sender ok
rcpt to:[email protected]lstrom.com550 5.7.1 [email protected]rom.com... Relaying denied. IP name lookup failed