Imagine you are using your favorite e-mail reader application to download messages but you don't want them to be removed from the server after being fetched. So you set the application preferences to not delete messages. Once done you will find out that each time you connect to your POP or IMAP server, only new messages are downloaded. Already downloaded messages are automatically discarded so you never get duplicates. POP protocol doesn't let you mark messages as fetched or not. It is the responsibility of the e-mail reader software to maintain a pointer and start fetching from last position.
eMail Bounce Handler also works that way but also lets you reset (set to 1) that pointer in case you want to restart processing the whole mailbox for example after adding or modifying a rule.
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