I just managed to do this in a very round-about way on a fluke. Was so happy to have discovered it. Here's what I did:

1. Go to inbox
2. Click on "Unread" in left hand column
3. At the top, you will now see the number of unread emails and an option to "refine" your search. Click on "refine"
4. A box will pop up..here, you will change the search option that says "unread" to "inbox"...you will also change the Dates option to "custom" and enter in as large of a span of dates as you think you have in your email inbox (format yyyy/mm/dd)
5. This should pull up almost all of your emails and allow you to click the little select all box to "move" to transfer them over to another folder as you normally would for a smaller amount. It may take a long while for it all to transfer and there may be a few hundred left in the inbox at the end which you can just transfer as usual.

Good luck!

I just managed to do this in a very round-about way on a fluke. Was so happy to have discovered it. Here's what I did:

1. Go to inbox
2. Click on "Unread" in left hand column
3. At the top, you will now see the number of unread emails and an option to "refine" your search. Click on "refine"
4. A box will pop up..here, you will change the search option that says "unread" to "inbox"...you will also change the Dates option to "custom" and enter in as large of a span of dates as you think you have in your email inbox (format yyyy/mm/dd)
5. This should pull up almost all of your emails and allow you to click the little select all box to "move" to transfer them over to another folder as you normally would for a smaller amount.

It may take a long while for it all to transfer and there may be a few hundred left in the inbox at the end which you can just transfer as usual.