The best action to take immediately after clicking "Send" if you forget to add something in the email body is:
3. Click on "Undo" to edit the email.
Many email clients offer an "Undo Send" option that allows you to retract the email for a short period after sending it, giving you the chance to make edits and resend it. If your email client supports this feature, it allows for a quick fix without creating a new email.