Please follow the following steps to enable SMTP authentication in Macintosh OS X MAIL email client :-

1. Open MAIL.

2. From the MAIL menu, select Preferences. Select the Accounts icon and 
click on your email account under the Description heading. Click on the Edit
button on the right.  The Account Information appears.

3. In the Outgoing Mail Server section, click on the double arrows and select
Add Server.. from the menu. The SMTP Server Options box appears.

4. In the Outgoing Mail Server box, enter your SMTP server name.

5. In the Authentication box,  click on the double arrows and select MD5 
Challenge-Response.

6. In the User Name box, enter your email address. In the Password box, enter 
your password for this email address.

7. Click OK to return to the Account information page. In Outgoing Mail Server section, 
click on the double arrows and select Your-smtp-server-name . Note that it will have 
your email address listed.

8. The Outgoing Mail Server entry should now show Your-smtp-server-name followed 
by your email address.

9. Click OK to close and then close the Accounts box by clicking the red button.

10. SMTP authentication has been enabled.