After a problem with our Verizon email accounts, during which I had to change the password on one account, that account could not send mail with the Apple mail app.
Yes, we changed to password on the account settings. But what I discovered, a day later, ws that there were two places to update your password.
The account screen password field is the obvious placeto update. But you have to go into the SMTP server settings and update your password there as well!
Finally, after much angst, missed communications and the loss of a few hairs (mine), my wife can use her email.