https://dns-lookup.whoisxmlapi.com/api
DNS Lookup API to check MX, NS, etc. server records | WhoisXML API
Our DNS checker API helps users check MX, NS, and other server records via API calls with outputs in JSON or XML. Get started with 500 free API queries.
https://mxtoolbox.com/
MX Lookup Tool - Check your DNS MX Records online - MxToolbox