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Firstly, do you delete them from the server when you download to your tablet? If not then you've not lost them at all. So it depends on how your account is set up.
Secondly, where is K9 storing your mail on the tablet? When I last used K9 I set it to store messages on my sd card (with the amount of internal storage that phone had I had little choice). Stuff on sd is easy to recover, and may well still be there. But I guess the default is just to store mails as app data, in which case they'd be in a place you can't access without root and there's probably nothing you can do.