Undeliverable: Request Id ##xxxx## is appended with requester reply

Undeliverable: Request Id ##xxxx## is appended with requester reply

Hi Guys,
 
What could be the possible reason why do we received message like this on Service Desk? and it is flooding the request on Service Desk. Any response would be appreaciated.
 
"Subject
Undeliverable: Request Id ##5002## is appended with requester reply.
Description
Your message
To: email@domain.com
Subject: Request Id ##5002## is appended with requester reply.
Sent: Wed, 23 May 2012 14:54:50 -0700
did not reach the following recipient(s):
email@domain.com on Wed, 23 May 2012 14:54:51 -0700
The e-mail account does not exist at the organization this message
was sent to. Check the e-mail address, or contact the recipient
directly to find out the correct address."









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