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:
The issue is SMTP server-related. Make sure your Jira instance has an SMTP server configured. Read more here.
Outgoing emails are disabled. Make sure that outgoing emails are enabled on the Outgoing Mails page. Read more here.
Something's wrong with the add-on's 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.
The user is not permitted to send manual emails. Permissions or conditions are configured in the Configuration tab of the Email This Issue admin page. Read more here. If you are not a member of the configured Project Roles or Groups configured or the issue does not fulfill the JQL condition, the button remains hidden to you.