“The Message can’t be moderated because the approval system is too busy and can’t accept messages now” when emailing to a moderated distribution group.

Scenario: You receive the following bounce back when sending to a distribution group that has moderation enabled:

Delivery has failed to these recipients or groups:

The Message can’t be moderated because the approval system is too busy and can’t accept messages now. Please try resending this message later, or contact the recipient directly.

Resolution:  The arbitration mailbox that the distribution group is using for moderation is having problems processing the message or located on legacy Exchange servers that is no longer accessible.  To resolve this,  set the arbitration mailbox to another arbitration mailbox.

To determine the arbitration mailbox that the distribution group is currently using, run this:

get-distributiongroup “All Users” | Select ArbitrationMailbox

To determine the arbitration mailboxes in the organization to choose from:

get-mailbox -arbitration   

-or get-mailbox -arbitration -domaincontroller RootDCName

Now set the new arbitration mailbox on the distribution group:

Set-distributiongroup “All Users” -arbitrationmailbox “SystemMailbox”be42da……..02ed2}”