MAC-Eudora6
Open the Eudora mail application. Click on the Eudora drop down menu and select "Preferences...".
Select the "Sending Mail" option on the side menu. Ensure there is a check mark in the "Allow authorization" box. Check that the mail server is mail.mydomain.com (where mydomain.com should be replaced by the your full domain).
Once all settings have been confirmed, click on the "OK" button in the bottom right hand corner of the window.