When users were sending emails to a contact group, it would bounce immediately with the following error message:
From: System Administrator
Sent: Monday, August 14, 2017 3:14 PM
Subject: Undeliverable: Email subject
Your message did not reach some or all of the intended recipients.
Subject: Email subject
Sent: 8/14/2017 3:14 PM
The following recipient(s) cannot be reached:
'Recipient 1' on 8/14/2017 3:14 PM
This message could not be sent. Try sending the message again later, or contact your network administrator. The client operation failed. Error is [0x80004005-0x80004005-0x000501].
'Recipient 2' on 8/14/2017 3:14 PM
This message could not be sent. Try sending the message again later, or contact your network administrator. The client operation failed. Error is [0x80004005-0x80004005-0x000501].
The issue was the related to throttling settings, and the distribution list was over the 24-hour limit.
To fix the issue, you can create and apply a new throttling policy to the affected mailboxes:
New-ThrottlingPolicy -Name policyname -RecipientRateLimit Unlimited
Set-Mailbox -Identity username -ThrottlingPolicy policyname
Replace ‘username’, ‘policyname’, and ‘Unlimited’ with the desired names and values.