So... everything was fine and then suddenly you can't send mail... even worse... you have been locked out of the server!
Unfortunately your settings in Mac Mail can get messed up whenever the automatic software updates are run on your computer. The only thing we can suggest is for you to update the settings again, making sure you turn off the “Automatically detect and maintain account settings” option.
- From within Mac Mail Click Mail, Preferences, then Accounts
- If you have more than 1 account, select the account on the left of the window.
- Near the bottom of the window, look for Outgoing Mail Server (SMTP)
- Click the drop down arrow and select "Edit SMTP Server List..."
- Click the Advanced button in the middle of the window and make sure Authentication is set to "Password."
- If it's there, turn off the “Automatically detect and maintain account settings” option
- Make sure you have entered your full email address and it's password.
- Ensure that you have selected "Use custom port", and enter 465.
- Ensure that "Use Secure Sockets Layer (SSL)" or "SSL/TLS" ticked.
- Click OK to close the edit window.
- Enter your password for your INCOMING SERVER again (important!!!).
- Close the window making sure that you save your changes.
- Quit Mail and then re-open it.
...another option is to avoid the continuing hassle of the Mac Mail app and use Thunderbird instead.