If an issue has been created from a particular email from Outlook, are replies to this email automatically added as comments?
No, they won't be added as comments automatically, you need to add them manually. But we are going to improve this step to pre-select the related issue.
Depends on your use case, but to automatically add comments you can also leverage built-in features of JIRA as explained here: https://confluence.atlassian.com/display/JIRA/Creating+Issues+and+Comments+from+Email or use plugins like JEMH: https://marketplace.atlassian.com/plugins/com.javahollic.jira.jemh-ui
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