You have configured Exclaimer Cloud - Signatures for Microsoft 365, and you notice that signatures are not applied to Out of Office emails or automatic replies.
When automatic replies are sent from Microsoft 365, MAIL FROM: information for the envelope is set to <> (that is, there is no MAIL FROM: information).
This means that automatic replies trigger an exception in the transport rule that identifies messages to send to Exclaimer Cloud; these messages are not routed to Exclaimer Cloud, and therefore, no signature is applied.
The MAIL FROM: information is used by Exclaimer Cloud to determine the correct Microsoft 365 tenant to return the email to.
If the emails with missing MAIL FROM: information cannot be attributed to the correct Microsoft 365 tenant, the delivery will be attempted via an empty sender tenant.
Sender Policy Framework (SPF) authentication and other checks will fail, causing the email to be rejected as spam. You will also not be notified of the rejection due to the missing MAIL FROM: information.
Unfortunately, it is not possible to use Exclaimer Cloud to apply signatures to Out of Office emails or automatic replies at the moment.
It is, therefore, necessary to have the Mail flow rule Prevent Out of Office messages being sent to Exclaimer Cloud enabled within your tenancy.