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.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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