Relevant Products: Exclaimer Cloud - Signatures for Office 365
In some instances, messages that are redirected to Exclaimer Cloud using the transport rule are not delivered to the recipient.
When a Non-Delivery Report is issued to the sender, it reports an 'Invalid message content' error, as shown in the example below;
This same error will also be displayed if you track the message using the Office 365 Message Tracking option.
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:
Here, the <email@example.com> placeholder represents the email address of the user.
Microsoft recommends that the JournalingReportNDRTo recipient is configured to be an external email address, or that a dedicated mailbox is created which has no transport or mailbox rules applied.
Please note that this solution applies even if message journaling is not used in your environment.
If the 'Invalid message content' NDRs only occurs when sending internal emails to distribution groups, the cause may be related to the ReportToOriginator setting of the group itself.
Please check our Knowledge Base article below to resolve this: