Inbound email processing can match a reply by using the watermark, the reply email's 'In-Reply-To' header, or the record number. See https://docs.servicenow.com/bundle/istanbul-servicenow-platform/page/administer/notification/concept...
Inbound action processing
The current prefix defined in the 'sys_watermark' table is used to match the email content. So if your current custom watermark is "ABCD", and you receive an email containing a watermark 'Ref:XYZ012345', then that will not be recognized as a watermark when looking in the email body. However, you may not see any impact on email reply matching, because it is likely successfully locating the target record instead via one of the other methods.
If the other methods are unavailable - which is less frequent - your reply would not locate the target record via the watermark. (For instance, a bug in an email application could prevent the presence of the "In-Reply-To" header). Therefore it is possible under less frequent circumstances, that a reply containing the old prefix would not be correctly matched as a reply.
What to do to prevent this:
In Helsinki Patch 3 and later, or Istanbul or later, use the property, 'glide.email.prior_watermark_prefix' and specify your prior prefix there. This ensures that if an email arrives with a watermark, it will try to locate a watermark in the email content by either new or prior prefix. This will handle the admittedly infrequent situations described above and ensure your email is still seen as a reply, accurately locating the target record.
Once your watermark transition period is over, you can remove the property. The transition time is up to you - it just needs to be reasonably long enough so that you don't expect replies to be arriving with the older watermark prefix.
