Sorry to say that TIme Machine is designed by Apple to be an "all or nothing" application. It is not adjustable in any way so to only back up some files and not others. It can only back up "everything" on your Mac's internal hard drive. AND, once a file, such as an email, is backed up by Time Machine onto your external hard drive, there is no accessing and deleting just emails off of the backup. You would have to erase the entire external hard drive to get rid of the email backups. You obviously would need a more sophisticated back up application than TIme Machine for your needs.