Mail @ Issue (Send email to issue)
Mail @ Issue (Send email to issue) is JIRA Cloud plugin. It gives you possibility to attach emails as comments. Just forward your message to: firstname.lastname@example.org
Every issue will have unique email address. Forwarded email will be visible in issue 'Emails' tab.
You can search for issue by mail content using JQL:
issue.property[mail].body ~ "my query"