Our server engineering team was investigating an issue on a bounce back a user got and the whole team spent a lot of time researching on below message.
From: System Administrator
Sent: Tuesday, April 20, 2010 4:53 PM
Cc: Recipient Name
Subject: Undeliverable: Trainings
Your message did not reach some or all of the intended recipients.
Subject: RE: Trainings
Sent: 4/20/2010 4:45 PM
The following recipient(s) cannot be reached:
Recipient on 4/20/2010 4:53 PM
This message could not be sent. Try sending the message again later, or contact your network administrator. The client operation failed. Error is [0x80004005-0x0004b9-0x000501].
At the first glance it seems like a problem with permissions on recipient mailbox. However,the error "0x80004005-0x0004b9-0x000501" comes from trying to submit a message on the Exchange transport. The code 0x0004b9 is an ecNullObject from Store, indicating that the failure occurred because an object referenced was null. This implies that this user was having some intermittent networking issue that happened to hit at the time the message was submitted.
This behavior is by design and the error is expected when network failures occur. If your computer is connected via wireless or a WAN link, then random failures on the network can be common, and that would explain the behavior. Even on a local area network, RPC failures are not uncommon.
In general, this problem is likely caused by intermittent network connectivity issues, or possibly out-of-resource problem on the client machine.
I suggest getting back to the user and requesting them to re-submit the message if similar kind of problem occurs in future as well.