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

CTO @ Golden Five | CEO at LAExUG Foundation

Prabhat is 3 times Microsoft MVP Award winner. He is MBA in Information Technology and he is working as a CTO at Golden Five Consulting which is a Microsoft Gold Partner, MSP, T1CSP, and Education partner. He helps in designing, implementing, managing and supporting solutions for private messaging cloud, mergers, a collaboration between different messaging software and other migration & deployment projects for the following technologies Office 365, Azure, AWS, Exchange, SQL, ADFS, MFA, FIM, MIM and Directory services. He has worked for all big IT giants either as an employee or contractor where he has led the Global teams. He has started his career as Technical Consultant in Exchange 5.5 with Microsoft PSS and his exchange love never stopped & continued with 2000/2003/2007/2010/2013/2016/2019/O365. At one point in time, he was the only person to support EDS customers when Microsoft had closed all the supports for 5.5 and now for all old legacy Exchange versions.
He used to blog at, manages multiple LinkedIn and Facebook Groups. He also Owns MSExchnageGuru YouTube channel where he uploads all his records technical sessions. Don’t forget to check his PowerShell scripts which are making admins life easier. Prabhat can be reached at

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