You will lose any and all information associated with that account that resides inside the user's home folder or subfolders -- Mail stores mailboxes and email messages in the "Mail" folder inside the "Library" folder located in the user's home folder... so, yes, you would lose that information.
If you wish to retain that information, you can simply back it up. There are more than a handful of methods of backing up that will retain this information, or make it easy to retrieve.