When you configured Xink Microsoft server-side and your emails that were sent externally or outbound emails do not get signatures, this is possibly due to outbound filtering of Mimecast. These emails are not being rerouted to Xink. Hence, signature is not applied.
To address this, you need to configure your Mimecast transport rule in your Microsoft 365 to catch emails coming from our relay IP address depending on which datacenter you are in. See the IP table below.
You need to modify "Apply this rule" setting as shown below so emails are rerouted to Xink first before Mimecast.