Email problem

Hi…ok….so ages ago i had my email set so that the sender would be a particular email account of mine. For some reason..possibly since i updated….that disappeared and is now stuck on a different email account…..and when i click the sender section to type the other one manually….it pops up this same one with a check mark next to it…a check mark i cannot undo…….any suggestions on how i can even just stop a sender being the default at all?? Id rather type what i want every time than being stuck….thank you