Under "Email and sms activity" in the menu on the left under ''messages'', you can see what "happens" with the emails you send to customers. Here is a description of the various events:
Sent: An email has been sent to the recipient.
Received: The recipient has received the mail.
Opened: The recipient has opened the mail.
Bounce: The recipient's server has blocked the mail, but we are trying to send it again.
Blocked: The recipient's server has blocked the mail and we are not trying to send it again.
Deferred: The recipient's server has delayed receiving the email.