Earlier quoted context omitted.
You're using the Gmail model, where mail has labels and gets archived. I think a lot of people are using the other model, where email goes in folders and gets deleted. Living in the MS world, I struggle to use the Gmail model. I create an Archive folder and put it as a favorite folder. When I'm done with an email, instead of deleting it like Outlook wants, I stick it into the Archive folder. I constantly have to chec…
https://support.office.com/en-za/article/Introduction-to-Aut... Right click on the folder (Sent Items in this case). Select properties, go to the AutoArchive tab. Specify the rules (where to move it and when). AutoArchive gets executed periodically, but you can also force it through the File menu. File -> Info -> Cleanup Tools -> Mailbox Cleanup -> AutoArchive (probably a faster way but that's the way I found).
Much as how AutoRecover (Word) is not AutoSave (N++), AutoArchive is not Archive.
Archive is the manual process of dropping your mail into another folder when you've processed it from your inbox, in order to keep yourself organized. AutoArchive, as the link suggests, is an automated tool that will age mail from your archive folder (and rules-based folders) into older storage folders, for technical reasons - folder capacity is finite, but we can make more folders.
Confusion between the two is probably exacerbating the Inbox Zero crisis.