Depends on how old the OS build you're using is. If it's a post 10.1 MR build (4xxx and higher) you can set your accounts to hold them up to forever - depending on the account type. ActiveSync and IMAP, for sure. Pop3 accounts, maybe only up to 90 days.
But, you have to set how long to keep your emails when you first set up the account on the phone. And, I think you would have to set them up using the "advanced" option.
If your accounts are already set up, you will have to delete and re-set up the account all over again and change that option before saving.
Posted via CB10