In the case of the website "correct.email", for bounced emails, out of 20,000 tested they found around 30 emails they classified as bounced (which my server didn't consider bounced). So that means a regular SMTP server gets the same result within 0.15% (less than 1 percent).
Other than that, they listed a lot of what they considered "Spam Traps" which the other provider didn't consider as spam traps (not even one matching). Also they listed a lot of "catch all" email IDs (maybe 5 or 10% of the total tested emails) which after manually going through all were valid.
So I didn't get much use out of it, but hey, it was free...
I would like to try zerobounce, because the small tests i did there, they appended the first and last name in 85% of emails, and all looked accurate. In comparison zero bounce didn't find any bounced or spam traps... very confusing.