Normally this setting is found within Internet Explorer under Tools > Internet Options. Select the 'Programs' tab and check which software handles your 'E-Mail'.
If you do not wish to use Outlook Express for this task, select the appropriate one from the list.
In Outlook Express you might want to check under Tools > Accounts. Select the 'Mail' tab and check your email account 'Properties'. Also check the Connection (DialUp) settings under "Tools > Options > Connection" or alternatively from within Internet Explorer under "Tools > Internet options > Connections"