The problem with the autoarchive is it merely delays the decision of what to keep and what to throw away. Autoarchive should be disabled, and users should move messages into a .PST manually.
Autoarchive saves everything to the .PST regardless of it's value. When users have to move messages manually then they're more likely to filter the messages as they go, archiving what they need and discarding what they don't. This is easier to do when they're filtering a few at a time. It is a much larger task to filter the messages after the .PST has grown too large and requires immediate attention to several thousand messages. Then there's the additional time-consuming step of compacting the .PST after the users had cleaned it up.
I recommend adding the 'Size' column to each users' display, then teaching them how to sort on the column headings. Full mailboxes and .PSTs are not caused by the sheer number of messages but by the size of individual messages, usually due to bloated attachments. Sorting the messages by size allows the user to see the real troublemakers.
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