The documentation seems to suggest this is NOT possible:
This message handler creates a new issue, or adds a comment to an existing issue. If the subject contains an issue key, the message is added as a comment to that issue. If no issue key is found, a new issue is created in the default project.
https://confluence.atlassian.com/jira/creating-issues-and-comments-from-email-185729464.html#CreatingIssuesandCommentsfromEmail-Addacommentfromthenonquotedemailbody mentions three handlers that only comment, without create.
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