You use Office 365 Groups, where all conversations, replies to posts, and calendar events are delivered to the group inbox.
Some group members have subscribed to receive copies of group messages in their own mailboxes. Messages received in the group inbox have an Exclaimer signature attached, however messages delivered to subscribed individuals have no signature.
This occurs because the message return-path on the SMTP envelope has no return path information - it is marked as:
MAIL FROM: <>
In other words, copies of group messages sent to subscribed individuals have no sender information.
Without sender details, there is no way to determine which signature to apply, therefore these messages trigger an exception in the transport rule which identifies messages to send to the Exclaimer Cloud. This exception ensures that messages without sender details are not routed to the Exclaimer Cloud for processing - hence no signature is applied.
This is expected behaviour given the nature of group messages however, Exclaimer will be reviewing how to implement signatures for these messages.