In my instance of Jira I have a particular Issue type that I have hidden the Priority field for. This particular issue type also has a service that uses the CreateIssueHandler to automatically generate a new issue. I have discovered that creating a new issue via email actually sets a priority automatically, even though this particular issue type doesn't need a priority set. When a user receives an email reply that an issue has been created, they are shown the issue priority even though this issue type should not have one.
After looking at the source, it is clear that CreateIssueHandler will set a priority on all issues all of the time. I understand the reasoning for looking at the X-Priority header of the email to determine incoming priority, but in my case I wish you were checking to see if the Issue type even needs a priority. My usage scenario is more of a boundary case, but it would be nice if you considered all fields to be optional.
I have modified the source and am checking for my particular issue ID's, but if you could fix this in a future version that would be great.