In some instances, messages that are redirected to Exclaimer Cloud using the transport rule are not delivered. Sometimes a Non Delivery Report (NDR) is sent back to the user, sometimes not. When an NDR reports is sent, it reports an 'Invalid message content' error, as shown in the example below:
smtp.eu1.exclaimer.net rejected your message to the following email addresses: The email system had a problem processing this message. It won't try to deliver this message again. smtp.eu1.exclaimer.net gave this error: Invalid message content
This can also be seen in Office 365 message tracking.
The Microsoft Knowledge Base article below provides a solution for this problem:
In short, a dedicated mailbox must be specified as the recipient for Journal Report NDR messages. This is achieved using the command below:
set-transportconfig -JournalingReportNdrTo <email@example.com>
Here, the <firstname.lastname@example.org> placeholder represents the email address of the user.
Microsoft recommends that the JournalingReportNdrTo recipient is set to an external email address, or that a dedicated mailbox is created, which has no transport or mailbox rules applied.
This solution applies even if message journaling is not used.
If the 'Invalid message content' NDRs only apply to internal emails sent to distribution groups, the cause may be related to the ReportToOriginator setting. Please check our Knowledge Base article below to resolve this: