If an email address triggers a "soft bounce" (i.e. when the inbox is full), this address is assigned the status "Temporarily blocked". The first time this happens, this status is reset after 72 hours. Each time a new temporary block is set, the period will be extended by up to 75 days. After 5 temporary suspensions, the email address is permanently blocked.
If you suddenly have an increased number of permanently blocked email addresses, this may be because "dead" addresses have been sent emails in the last 5 months and have now been permanently blocked. Consider this process as list clearing.