ServiceNow finding a watermark from another ServiceNow

[Mariette Zaal]

How can we stop ServiceNow creating a new case when the following happens?

Our ServiceNow SNA receives an email from a customers ServiceNow SNB, there is a watermark from SNB at the bottom.

SNA can't find this watermark so creates a case and sends a confirmation email to the customer. 

The customer sends an update and we can see SNB's original watermark, our SNA's watermark and then a second SNB watermark

So at the bottom of the email it says

SNB123345

SNA456786

SNB234567

Our SNA searches for SNB234567 in the watermark table instead of SNA456786, can't find it so it creates a new case as ServiceNow always looks at the last watermark. How do I make our SNA look for the SNA watermark?

[Omkar Mone]

What if you don't read the watermark when it comes from SNB or replace it with SNA watermark before inbound action picks it?

[Mariette Zaal]

How do I get SNA to look for its own watermark and ignore the ones that don't match? Should I add code to look for SNA's custom watermark prefix?