There are two ways to confirm your email was sent:
BCC to yourself — by default, your email address is added to BCC on every outgoing email. When your recipients receive the email, so will you. You can remove yourself from BCC in the email settings if you prefer not to receive these copies.
Calendar view — sent emails are visually crossed out in your Calendar view, so you can see at a glance what has been sent and what is still scheduled.