If you are upgrading to a later version of Mac OS X, such as Yosemite, El Capitan, or Sierra, you may encounter issues in which you are unable to send or receive emails, or both in the default Mac Mail app.
This is generally due to some changes Apple has made in the way the Mail app works.
- Open Mail.
- Access the Mail menu, then click on Preferences.
- Select the account you are using on the left pane.
- Click the Advanced tab.
- Uncheck Automatically detect and maintain account settings.
- Save your changes, and access the Account Information tab.
- Find the Outgoing Mail Server section and click the drop-down menu. Click on Edit SMTP Server List.
- Select the outgoing mail server you're using and click the Advanced tab.
- Uncheck Automatically detect and maintain account settings.
- Save your changes. Reboot your computer.
At this point, email should be operating normally.
Tags: email, mac, mac mail, servers