TO enable SMTP authentication in EUDORA please folow the followinig steps :-

1. Open Eudora email client.

2) Click on "TOOL" menu and then click on "OPTIONS".

3) Under the Category heading on the left, click Sending Mail .

4) Select Allow authentication on the right side of the Options window.

You can also do this for your personalities under Tools>> Personalities.

Note: Eudora uses the login name specified on the Checking Mail configuration option,

which is in the format of username@yourdonain.com.

5) Change Secure Sockets When Sending at the bottom of the window to Never.

6) Click OK.

Your  "SMTP Authentication" has been enabled in the EUDORA.