The device can only hold so much. So all those mails you save are taking up more and more storage on the device. Most have a limitation on how many they can hold, and then they will start deleting the older ones to make room for new ones. I would suggest you save them off the device. Cloud account, computer, etc., that way you can free up room for the new ones.
And remember, if you lose that device, it is stolen, it dies,gets reset, etc., you will lose all of it. So backing up stuff off of the tablet is always a good idea.