We are currently on Jira Service Management 5.1.1. We are having a problem where a user will create an issue via email. A Jira user will respond from within Jira and the end user will receive the update on the issue. When the end user attempts to respond to that email, their email is not added as a comment, and it is not created as a new issue. Their reply email also does not show up in the Email requests processing log. This issue appears to be random, as sometimes the end user replies are added on as a comment.
