These are the possible statuses you may see when sending an email campaign:
Message has been successfully delivered to the receiving server. For example if their email address is firstname.lastname@example.org, the message has successfully been sent to Gmails server.
Message has been received and is ready to be delivered.
You may see the following drop reasons: Invalid SMTPAPI header, Spam Content (if spam checker app enabled), Unsubscribed Address, Bounced Address, Spam Reporting Address, Invalid, Recipient List over Package Quota
Recipient’s email server temporarily rejected message.
Receiving server could not or would not accept message.
Recipient marked message as spam.