Current State:

Currently , when "Allow all emails that contain a valid issue key to be added as a comment to the issue?" is set to "Yes" , customers who are not the request participant or reporter or member of organization added to the issue are able to add comment to the existing ticket through email. Hence , their account will be added to the ticket. Due to this, agents able to mention then from Jira. However , the mentioned customer will not be automatically added as "Request Participant"

Proposed Future State:

Automatically added as a Request Participant for them to view the issue.
 

Alternative (e.g. Third Party Plugin):

Agents need to mention them from the Portal. The customer will be automatically add as request participant.