Not every message to your list will be received immediately by all your subscribers. Some messages cannot be delivered because of problems with a subscriber's ISP. Other messages cannot be delivered because of a problem with the subscriber's email address. In either case, when we cannot deliver a message to a subscriber, they are considered bouncing.
A bounce message is just a record of our delivery attempt, and the reason we were not able to deliver it. When we receive a bounce message, the first thing we try to do is figure out what subscriber sent it. Usually, this is easy, but sometimes, the full email address of a subscriber doesn't appear in a bounce message. Assuming we find a subscriber in the bounce message, we next try to group the message into one of 3 categories:
User Unknown: The most common kind of bounce. It means the ISP does not recognize the user name (the part of an email address before the "@") of the email address. This could be caused by the user changing email addresses, a typo, or, rarely, a technical problem at the ISP. This is also the only type we categorize as a permanent, or "fatal, bounce.
Mailbox Over Quota: The list subscriber has too much mail in their mailbox, so the ISP stops accepting mail. This can be caused by a lot of list mail, a large attachment, a vacation, or an abandoned mailbox (especially from a free provider like Hotmail or Yahoo.)
Domain Error: Usually caused by a typo or a deliberately misspelled email address. Sometimes indicates a problem with an ISP.
Bounces we cannot classify are termed "unknown". Once a bounce has been matched with a subscriber and a category, it is stored, and a "bouncing" flag is added to the subscriber's account.
At the end of each day, we analyze all the bouncing subscribers. Your list has 2 thresholds, one for permanent bounces, and one for temporary bounces. This can either be a number of consecutive days, or a number of consecutive bounces. (The check for permanent bounces is lower, because subscribers who start bouncing "user unknown" almost always are gone for good.) First, we check how many bounces we've received for each subscriber. If they have not yet reached the thresholds you've specified, we ignore that subscriber. Then we make sure that subscriber has bounced all the list messages during that period. If any list messages have been successfully delivered during that period, we ignore that subscriber. Only subscribers that have bounced every list message during your set period are unsubscribed. Subscribers have their "bouncing!" flag removed after they have not bounced a single message for the period specified by your highest threshold.
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