Having said that, haveibeenpwned.com has a pretty solid reputation as explained by cryptomaniac above. I wouldn't worry too much about using it, but I certainly wouldn't use any other website along the same lines.
Apart of emails haveibeenpwned.com offers to check passwords on the matter whether they have ever been exposed to public. Notwithstanding the reputation of this online service when checking entered emails I would be reluctant to send it the query to check for pwned passwords despite they claim of utilizing the k-anonymity technique in their checking procedure.