If you have converted to gmail this will convince you.
You use Microsoft Office and download email off the web and onto your computer and it is stored and archived in Outlook. How do you back the emails up or transfer them to your next computer?
An excellent article with very easy to follow directions called Export and Backup Emails from Outlook to Gmail Online
I spent many hours trying to use that gmail loader software, failing every time feeling fustrated and defeated that Microsoft controlled my emails not me. I was finally able to do back up over a thousand emails from the last 6 years from 3 different email accounts that I had stuck on my old laptop in Outlook. Now my email is safely backed up and assessible from any computer.
*In the first place don't delete emails from web-based email if and when you download to Outlook. Leave a copy online
*Before backing up and transferring email to gmail, clean up and organize. Of course delete all email you don't need to transfer.
*Then organize email into folders so you can export it smaller batches and when you do transfer them it will make it easier to label and archive once it is in gmail.
*Also get your gmail inbox to zero before you start it will make it much easier to label and archive incoming email from outlook.
*Add an "imported" or a "backup" label when you archive in gmail.