There are several message history events that could be shown for a given message. Below is a brief description of each history event type.

Send - The most common event type this is registered when the message is sent to the user.

Deferral - Typically due to a minor email sending outage, this message has been queued to be sent later.

Hard Bounce - This is a permanent error where a mailbox does not exist and the message will never be able to be delivered.

Soft Bounce - This is a temporary error like the recipient email address has a full mailbox. Message delivery will be attempted again.

Open - When a user opens to view your email message. It's important to note that not every email client supports this functionality.

Click - A user clicked on a URL within your email message.

Spam - The user has reported this message as spam. 

Unsub - The user has requested to be unsubscribed from future emails.

Reject - The recipient email server has rejected the email message. There are many reasons for a possible rejection.