Soft bounce:

A soft bounce is a temporary failure of a valid address such as an inbox being full or out of the office autoresponders. Since bounces like these are only temporary, they are kept in your active subscriber list until the 5th time they bounce. After the 5th soft bounce, the address is quarantined in the Manage Bounces section and removed from your active subscriber list.

You are able to batch or individually add addresses back to your active subscriber list. However, please note that any address you add back into the system will still go through the same 5 bounce rule as indicated above.