"554 5.4.14 Hop count exceeded - possible mail loop" NDR for outgoing email that's sent to an on-premises application in Exchange Server

• Applies to:

  Exchange Server 2016, Exchange Server 2013

Symptoms

Consider the following scenario:

• You have a parent domain, contoso.com, that you have added as an accepted domain to an on-premises Microsoft Exchange Server 2016 or 2013 environment.
• You have an on-premises application server domain, such as app.contoso.com, that is a subdomain of the parent domain.
• The parent domain is configured to accept email messages from domains such as *.contoso.com.
• You try to send an email message to the on-premises application through Exchange Server.

In this scenario, the message cannot be sent. Instead, it loops between the Exchange Edge Transport server and Exchange Online Protection (EOP). Additionally, you receive a non-delivery report (NDR) that resembles the following:

554 5.4.14 Hop count exceeded - possible mail loop

Cause

This issue occurs because the Exchange Edge server cannot associate the SMTP address space for the application as a subdomain to the accepted domain. This is true even though the accepted domain is configured as a parent domain.

In this scenario, the subdomain would be part of the address space in the EdgeSync - Inbound to SiteName send connector.

Resolution

To fix this issue, follow these steps:

1. Add the subdomain as an accepted domain. To do this, run the following command:

   New-AcceptedDomain -DomainName app.Contoso.com -DomainType InternalRelay -Name app.contoso.com
   

2. To have the added accepted domain synced to the Edge servers immediately, run the following command:

   Start-EdgeSynchronization
   

   Alternatively, wait for the changes to be synced to the Edge servers.