To change incoming and outgoing mail server for outlook and outlook express: 

Select:  "Accounts" under the "Tools" menu.............

Make sure the "Mail" tab is selected then select the mail account and then click  "Properties"

(If the account does not exist, select "Add Mail...... and follow the prompts to create it)

select the Servers Tab and change the incoming mail (POP3) and outgoing mail (SMTP) to the match your domain address as shown in the boxes below. Substitute your domain name for "mydomainname.com".  Be sure to place a check in "My server requires authentication"