Apple Mail for Sierra includes a feature which attempts to automatically detect the server and login settings for any e-mail account which has been created. However, this can sometimes prevent you from being able to send or receive e-mail. If you have recently upgraded to or have created an account in OS X Sierra, then you will need to disable this feature.
The following steps will assist you in disabling this feature:
1. Open Mail.
2. Select Mail from the menu and click on Preferences.
3. Click on the Accounts icon and select your e-mail account from the list on the left-hand side.
4. Select the Server Settings button, located beneath the row of icons at the top of the window.
5. Disable both tick boxes labelled Automatically manage connection settings.
Additional options will now be displayed in the window.
6. Under Incoming Mail Server, ensure that the following settings are used:
- Port: 110 (for POP) / 143 (for IMAP)
- Use TLS / SSL: this must be disabled
- Authentication: Password
7. Click on the button labelled Advanced POP Settings / Advanced IMAP Settings.
8. Place a tick in the box labelled Allow insecure authentication and click OK.
9. Under Outgoing Mail Server, ensure that the following settings are used:
- Port: 587
- Use TLS / SSL: this must be disabled
- Authentication: Password
10. Click on the Save button to save the changes.
