In Mac OS X Yosemite a new feature has been introduced that overrides settings for mail server connection.

Having this feature enabled leads to authentication issues since default settings might not be suitable.

To disable the feature in Mac Mail do the following:

  1. Go to Mail > Preferences.
  2. Choose the mail account and click Account Information.
  3. In the Outgoing Mail Server drop-down menu choose Edit SMTP Server List...

    Account Info

  4. Choose a server and switch to the Advanced tab.
  5. Uncheck the box Automatically detect and maintain account settings.

    SMTP Servers

  6. Choose the account again and click Advanced.
  7. Uncheck the box Automatically detect and maintain account settings.

    Advanced Settings