When the user sending the email is not admin and is not the submitter of the incident, the email's content is not attached to the open incidents log. Instead a new incident is created.
The sender's email-address-domain is trusted and the customer-field's contact-email-address is not the same but has the same domain name.
In this case we would expect that the emails content should be logged with body's content and attach email to the Incident addressed in email's subject.
Service Management
Service Desk Manager
This is neither a bug nor an enhancement. This is working as per the Service Desk design.
Customer is trying to update a ticket that has Affected End User(ex: User1) from an email address (that is not an Admin/ Analyst/ User1 email address).
Service Desk honours Licensing and any ticket update can only be done by a licensed user(Admin/Analyst).
Customer is using an email address belonging to an Employee access type which is an unlicensed user(who can only view/update their data) and since the MailboxRule has Create/Update Object Action, it creates a ticket as update of other affected end user ticket is not possible for unlicensed user.
Hence this is working as per design and we cannot deviate licensing restrictions.