Non-Delivery Report: [500 5.5.1 Invalid or unrecognized command in this state] – Exclaimer Knowledge Base

Scenario

You have configured Exclaimer Cloud for your Microsoft 365 tenancy and you notice some messages are rejected with a Non-Delivery Report (NDR), similar to the example shown below:

xx1.smtp.exclaimer.net gave this error:
Invalid or unrecognized command in this state.

Reason

Typically, this scenario occurs where multiple hosts are configured for an MX record. Occasionally, Microsoft may change the MX record associated with your default domain while undertaking maintenance.

Resolution

We recommend: You change the Mail Routing options within your Exclaimer Cloud subscription to a domain that only has a single host specified on the MX record. In many cases, this can be resolved by changing your domain from xxxxxxxxxxx.mail.onmicrosoft.com to xxxxxxxxxxx.onmicrosoft.com (removing "mail.").

Follow these steps to resolve the issue:

Log into the Exclaimer portal (portal.exclaimer.com) and Launch your subscription.
From the left-hand pane, click Configuration, then select Manage Mail Flow.
In the right-hand pane, under Mail Routing Domain, are the relevant options.
In Domain Name, enter the Microsoft 365 domain name with a single host. This setting is important, and an incorrect domain can result in mail flow issues.

Example:
Click SAVE to save the changes made.

NOTES: The MX record for the Domain name (in the Mail Routing settings) should specify a single Exchange Online Server for your Microsoft 365 tenancy.

- To check the MX record for your own Domain name, use a DNS lookup tool, such as MXToolbox.

Example showing Exclaimer's domain: