Send Email dialog is available on issue view by clicking Send Email button:
In this dialog you can create your own email or select existing project mail template. Specify To, Cc and Bcc recipients, subject and body of your email and click Send button.
In every recipient-like field (To, CC and BCC) it's possible to select JIRA user (like in the above exmaple) or to enter any email address (example@example.com).
Right upper corner of the screen message appears as a sign of successful email sending:
