Let's start by turning off address bar search.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste keyw and pause while the list is filtered

(3) Double-click the keyword.enabled preference to switch it from true to false.

Then test again with a nonsense URL. Do you get the expected error?

If instead you get search results for that or for any one word (space-less) search, then those results are a feature provided by your DNS service provider, which often is your ISP. Check the results page for any way they might offer to discontinue the service and just get plain error messages.

Note that it is still possible that someone has reserved a domain that is a mistyped version of a popular website, so you should always be careful anyway.

Thank you for responding. After following the suggested steps, I still get the search results. I will try ATT support.

I am aware of that. Thank you for your suggestion.

If AT&T won't disable the Yahoo results for "server not found" results, you could use Google Public DNS or an alternate DNS provider which doesn't serve search results.

Thank you for your suggestion - problem solved.