Whether or not it cost them anything, the fact is that--according to what you have written--you made it your employer's account. You voluntarily transferred it to your employer. The fact then that you may not have used it for business, but only for personal purposes doesn't change the fact that by your own voluntary actions, you made the employer the owner of your account. So yes, they can keep this account--because by what you  did, it is not your personal email account, but because their account.