The Incident Email Listener is used to handle communications from the user for Incidents. If the user sends an email to the Inbox that is monitored by the Incident Email Listener it will either create a new Incident or add the communication to an existing Incident.
The Subject line is used by the Email Listener to determine whether it is an existing Incident or if it is a new Incident. For the Incident to be updated the Email Subject Line has to include the ID of the Incident along with the Object Mapping Keyword. By default this is "Incident".
If the email listener receives an Email Subject with a Object Mapping Keyword it does not recognize this article details what happens.
If an Inbox is configured for Incident, only Incident, Task & Approval will be updated by ID in subject, other types will not be recognized and result in a new Incident.
If an inbox is configured as 'Other', since it's generic, the email listener will try to match the ID using field specified in the Unique Key.