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Atlassian adds the original mail address as a CC to any email that is forwarded to the system. We forward most mail to the cloud. For example, aascsys@syr.edu is forwarded to aascsys@su-jsm.atlassian.net. Atlassian will auto-add the original email address to the email as a CC. And we allow all CC addresses to be a Request Participant for that ticket.
ReferenceAtlassian has a public reference page: https://confluence.atlassian.com/jirakb/email-requests-rejected-due-to-jira-mail-loop-filter-1189798024.html
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