There are four different values that display based on different actions to the Mailing. That is, you could see Received, Sent, Bounced, and Pending.

1. Received: when the envelope is opened in Acoustic Campaign, it means the email has been opened and the SFDC status shows as received.
2. Sent: when the envelope is not opened in Acoustic Campaign, it means the email has not been opened and the SFDC status shows as sent.
3. Bounced: when there is a hard bounce on a Mailing. Hard bounce turns into undeliverable and immediately gets added to the master suppression list.
4. Pending: soft bounce retries up to 24 hrs. There is a setting by default and after three consecutive soft bounces over 21 days it turns into undeliverable.