If the e-mail addresses are MSN, Hotmail, or Live acounts, you can use the information here:

http://windows.microsoft.com/en-US/hotmail/hotmail-outlook-faq

Else, you will need to get in touch with the e-mail address domain provider to get the settings.

If you find that the response does not apply to the issue you raised. Try providing details of what has been happening. That way, we can pinpoint what the issue is, provide the appropriate solution, and avoid a guessing game.