https://mxtoolbox.com/

MX Lookup will list MX records for a domain in priority order. The MX lookup is done directly against the domain's authoritative name server, so changes to MX Records should show up instantly. You can click Diagnostics, which will connect to the mail server, verify reverse DNS records, perform a simple Open Relay check and measure response time performance. You may also check each MX record (IP Address) against 105 DNS based blacklists. (Commonly called RBLs, DNSBLs)