OK, so let's say I export all emails from Gmail from 2004 til today and I want to import them into Outlook.com. How would I go about doing this?
* download & open up thunderbird
* Add 2 accounts: your existing gmail account, and your new outlook.com account
* select all email in the inbox of your gmail account, ctrl+c [this will take a while]
* open the inbox (or other folder) of the outlook account, ctrl+v
Personally, the largest migration I've done using this method was ~1GB, but should work for larger accounts as well. Hope this helps!