Exchange Hybrid DL Moderation Issue: NDR ‘550 5.7.106 APPROVAL.InvalidContent

This blog explains the resolution for the NDR with error “‘550 5.7.106 APPROVAL.InvalidContent;”


4 Exchange 2016 in High Availability

2 Edge Exchange 2016

             2 Hybrid Servers for the Mailbox Migration


    1 On-premises Distribution Group

    5 members where 4 mailboxes in Exchange Online and 1 mailbox in Exchange On-premises

    Approver User mailbox is in Exchange Online


    When an on-premises user is sending an email to this DL.

    Approver receives the message and approves it.

    But receives the following NDR.


Delivery has failed to these recipients or groups:

Microsoft Exchange (Guid}
Your message wasn’t delivered because the recipient’s email provider rejected it.

The following organization rejected your message:

Diagnostic information for administrators:

Generating server:

Remote Server returned ‘550 5.7.106 APPROVAL.InvalidContent; Invalid content. [Stage: OnCreatedEvent][Agent: Approval Processing Agent]’

Original message headers:


Important to observe the following:

  1. Mail flow is working fine between Exchange Online and On-premises.
  2. Edge Transport which is the 1st receive is not sending the NDR
  3. NDR is coming from Exchange 2016 mailbox role servers.

We ran the following command for all 4 Exchange 2016 Mailbox Role server which fixed this issue.

Get-ReceiveConnector “Default Servername” | Set-ReceiveConnector -TlsDomainCapabilities “”

This solution has been mentioned in the following forum post.

Hope this helps!

Prabhat Nigam (@PrabhatNigamXHG)

CTO @ Golden Five Consulting | CEO @ LAEXUG Foundation

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