What are Unsent messages?
You may have noticed that when viewing your report page for sent engagements that there is a sub-section labeled "Unsent".
Unsent messages are typically reported when emails are sent to those email addresses which may have a full mailbox (soft bounce) or an unavailable mailbox (hard bounce).
In the case of the former, the email is not able to be sent at this particular time, but the user is not unsubscribed from your list.
In the case of the latter, the email is not able to be sent due to a permanent issue so the user is automatically unsubscribed in order to keep your sending domain in good standing for future sends.