This is the Email button in the issue screen that allows you to send emails manually:

There may be several reasons why this button is not shown:

• SMTP server: make sure your Jira instance has an SMTP server configured
• Disabled outgoing emails: make sure that outgoing emails are enabled in the Outgoing Mails page
  
   
• Add-on status: make sure Email This Issue is installed and enabled, as well as all modules are enabled and a valid license key has been entered
   
• Permissions or conditions are configured in the Configuration tab of the Email This Issue admin page. Check section "Permissions and Condition for Sending Emails Manually" in this page. If you are not a member of the Project Roles or Groups configured or the issue does not fulfill the JQL condition, the button remains hidden.