Hi,
Assuming that we are not talking about your google account that you used to set up the phone (in which case The_Chief's answer would be my first guess)... could you perhaps tell us where (i.e. in which application) you are entering the email? If it is just specific app then resetting its data would probably cure the issue. Also, if you are using a specific keyboard, you could try clearing the data of it. For example swiftkey stores previously entered words to be suggested again later.
Regards,
Eric.