I am really sorry to say that the Nook Tablet email app is so bad that you could buy an old, non Smart Phone, and have a better mail client.
As far as I understand the tablet retrieves the most recent 25 emails so older emails will still be there, if using Imap for sure, but you won't be able to see them with the Nook Tablet.
Another issue with the Nook Tablet's email is that it does not sync offline actions so if you ever delete while offline, that won't sync.
Overall I don't think the mail program could be used as a primary mail program. I find it so bad that I am not using it at all and using my Iphone's mail client... even with the smaller screen way more usable.
My primary email address is a POP server. I've tried a few different things, but it always gets rid of all my old emails to get new ones. It's too bad, because I would really like to keep some of those emails and attachments for later reference as I do on my PC. Perhaps they'll update it in the future or something.