In Junk E-mail options, the option “Warn me about suspicious domain names in email addresses (recommended)” is unchecked and unavailable (grayed out).
How can this option be enabled again?
It sounds like you are still using Windows XP with Internet Explorer 6 as this option is only available when you have Internet Explorer 7 or later installed.
If you for some reason need this option available to you but are not ready yet to install Internet Explorer 7 or later, then you can install IE7 or IE8 and then remove it again.
The option will still remain available to you in Outlook even after uninstalling IE7 or IE8.
Note: Internet Explorer 8 is the last version of Internet Explorer for Windows XP.