Currently, there is no method to identify issues that are created in Jira projects by mail Handlers if the user who raised the issue is within the Jira site.

Jira adds "Created via e-mail received from: test@test.com" only when the issue is raised by external users who are not part of the Jira site or if its the reporters themselves.

Suggestion:
We'd like them to have a unique identifier - field or label  so we can filter out issues created by Mail handler.