Suddenly cannot send emails on iPhone xr

As of a few days ago (I found out today that an email did not send from 5/30/19, plus usually do not use my iPhone to send emails), I can receive emails but cannot SEND emails. Keeps telling me " username or smtpauth is not correct". I checked and everything is the same as always. Also, I just installed the newest update manually (instead of waiting for auto update), and still not able to send email. Can anyone help?