Remove SMTP servers in macOS Mail

In macOS Mail, to remove an SMTP server you've added to your list of outgoing servers:

1. In Mail, from the Mail menu, select Preferences....
2. In the preferences window that appears, click the Accounts icon.
3. Click the Account Information tab.
4. On the right side of the window, near the bottom, from the Outgoing Mail Server pull-down menu, select Edit SMTP Server List... or Edit Server List....
5. In the sheet that appears, select the server(s) you wish to remove, and click the - (minus sign) button.
6. When you've finished removing servers, click OK.

The changes should take effect immediately.