[SC-Help] Mail not delivered

[smims at sunstoneinc.com]

We are having problems with a couple of our partners receiving the 
following message when sending email to our address.  How do we find out 
what's causing this delivery failure?

From: Mail Delivery System [mailto:MAILER-DAEMON at ironport.qrs.com] 
Subject: Delivery Status Notification (Failure)

The following message to was undeliverable.
The reason for the problem:
5.4.7 - Delivery expired (message too old) ''

Steville Mims
smims at sunstoneinc.com