When an email address is undeliverable due to a bounce, the entry is flagged in your account under the Subscriber Information for that subscriber.
We will continue to send messages to a subscriber until they have been undeliverable more than 3 times in more than a 7 day period. If a subscriber has bounced 3 times in that 7 day period, they will be unsubscribed on the next bounce after the 7 day window is concluded.
This process will reset every 60 days. So, if a subscriber is undeliverable 3 times in a 7 day period and then does not bounce for another 60 days, they will not be unsubscribed. After the 60 days have ended, a subscriber will need to be undeliverable more than 3 times in a 7 day period to be unsubscribed.