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mapList: A list of all Result Maps that should have issue association resolved based on the Message ID.
onIssueIDField: The ID of the custom field the Message ID of the email will be stored on the issue.
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The jemhThreadUtils class also allows for issues to be associated via an external ID. This allows values other than the Message ID to be used. As an example, a value from the Subject. Typically this requires some additional configuration as the external ID must be identified in the email manually. For a detailed walk through of this scenario please see:
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example in the Subject of the email, or via regexp.