"554 5.4.14 Hop count exceeded" NDR for outgoing email sent to an on-premises application in Exchange Server

Symptoms

Consider the following scenario:

• You configure contoso.com as a parent domain.
• You add the contoso.com parent domain as an accepted domain in 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.
• You configure the parent domain 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 can't be sent. Instead, it loops between the Exchange Edge Transport server and Microsoft 365. 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 can't 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 is part of the address space in the EdgeSync - Inbound to SiteName send connector.

Resolution

To fix this issue, use the following steps:

1. Add the subdomain as an accepted domain. Run the following command:

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

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

   Start-EdgeSynchronization
   

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