I came across a problem this week whereby emails being sent from a supplier and processed by our ServiceNow instance were updating the wrong tickets.
After some information gathering it became apparent that the supplier also used ServiceNow!
The problem arose because our instance was processing the inbound email and matching the watermark (generated on the supplier instance) with a watermark local to our instance, this meant the wrong ticket was being updated.
Out of the box ServiceNow uses the watermark prefix: MSG, this can be amended to a prefix of your choice (preferably different from the suppliers!). Changing this ensures the correct inbound action will be triggered, you can make the change by browsing to:
System Definition -> Number Maintenance -> Amending the record for “sys_watermark”.
I hope this proves useful to someone!