There is a bug in the macmail software. you have to change the outgoing mail server settings to manual, and the make the settings to 465, outgoing server requires authentication. and password. Make sure you erase the dots currently in the password field and then re-enter the password. I used to beta test for Apple, and I know that bug well. I actually reported it when they went from 10.4 to 10.5, a decade ago, and they STILL haven't fixed it. Also, ensure, prior to doing all this, delete everything sitting in the outbox.