When Apple Mail is running
- select "Preferences" from the "Mail" menu
- select the desired mail account
- on the "General" tab in the "Outgoing Mail Server (SMTP)" section click on "Server Settings..."
- from the "Authentication" select box choose the desired mechanism, usually "Password" is the right choice
- enter your username and password for the mail server in the two fields below
- click on "OK" and close the Accounts configuration window.