follow these steps:
1.Open Mail by going to the Dock and clicking the Mail icon or by navigating to the Desktop > Macintosh HD > Applications > Mail.
2.Go under the Mail menu and then select Preferences.
3.Click Accounts and choose the active account.
4.At the very bottom of this panel, you'll see Outgoing SMTP Servers. Click the Server Settings button (as shown above).
5.The following panel will appear (below). Click the server you wish to remove and click the Remove Server button. That server will now be removed from your SMTP listings and once you close the Preferences box, Mail will ask you if you want to save the settings and you should choose OK.