When you archive a piece of mail, it'll bring the piece of mail out of your "All Mail" view and into the "Archived Mail" view section in your Stable Dashboard. This is a great way to organize mail and designate that you've already dealt with that piece of mail!
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Archived mail will never get deleted — rather, it's a way of clearing it out of the other dashboards and marking that you no longer need to take action on the mail.
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The digital copies of the mail will be available to view forever. However, physical copies of your mail will be shredded after 30 days, so keep that in mind if you do need a physical copy of the mail.