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Sending emails from Jira & sharing the email and conversation is a brand new feature for Office 365 & Jira Cloud. It allows you to send emails from Jira and have the whole conversation including replies shared with everyone accessing the Jira issue.
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User guide: Send emails from Jira
Send emails from Jira
User guide: Sharing a conversation / email
Sharing an email
Admin settings: Further information as well as the technical reference
Configure this feature
Sharing Office Items
Detailed explanation of the possible sharing options can be found here:
Sharing options