...
The objective is to balance the negative effects of having a "dirty" list with the problem of unsubscribing recipients that should not be unsubscribed (e.g. mailbox temporarily full).
A carefully calibrated system taking into account industry standards will temporarily unsubscribe recipients who, in a given period of time, will return a number of consecutive bounce for the same reason, without having had recent activities. This will let customers to automatically follow the best practices and avoid to continue sending to email addresses that can not receive communications.
At the end of the temporary unsubscribe the recipient will be automatically re-entered as subscribed. If a further bounce for the same reason is returned, the recipient will be unsubscribed temporary for a period of time slightly longer.
Subsequently, if the in box of the recipient continues to be unreachable it will be classified as "permanently unsubscribed to further bounces.
Although it is possible at any time to force the re-submitting it is not reccomended to not do it untill it has been clarified and resole the motivation for not delivering the message. Otherwise the recipient would be unsubscribed within the further sending.
Hard bounce
When a Hard Bounce is received (e.g. 550 5.5.1 "Mailbox not found" or "Unknown user"), that recipient is not included in future mailings. This is true after the first HB and leads to better deliverability for the rest of the list. This task is performed automatically.
...