From the main screen open the 'Tools' menu and then select 'Account Settings
Select your email account from the list.
Click the 'Change...' button
change the 'Outgoing mail server (SMTP)' to the SMTP server that was supplied to you by your ISP
If your smtp requires an authentification, click on Click the 'More Settings...' button
Select the 'Outgoing Server' tab.
Tick 'My outgoing server (SMTP) requires authentication'
Select 'Log on using'
Enter the AuthSMTP username and password that was supplied to you.
Click the 'Next >' button then the 'Finish' button