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During Creation
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Reporter
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- can populate the reporter if the sender has a registered JIRA account
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- can populate a TEXT single line custom field with the email address of the sender if the sender does not have a registered JIRA account
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- can populate a TEXT single line custom field with the "personal" part from the sender email address if it is present
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Participants
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- can populate Watchers (for JIRA registered addresses) or User Picker custom fields (for JIRA registered addresses) with _participants_ (the CC or TO list you refer) if configured in the Profile to do so. JEMH does not differentiate between TO: and CC: it did work this way long ago, but frankly users were not able to understand nuances introduced, hence simpler model.
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- can populate a TEXT multiline custom field with the addresses of other participants who do not have a registered JIRA account.
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Commenting
JEMH:-
- can enforce security through trust such that only 'email' participants who are already referred in the Sender/Participant custom fields are able to comment on the issue. When such comments occur, any _additional_ participants added by that user when replying to the issue, will also be added to the issues Participant custom field (or Watcher, or Custom Field if related to a JIRA user)
Turning off that email-user security is possible, but means anyone can comment on any issue, subject to visibility to the Default Reporter.