What MxToolbox Diagnostics and Blacklist Checks Tell You About Your Mail Server

Updated

MxToolbox's MX Lookup tool can tell you three things about your mail server: whether it is reachable and responding, whether its reverse DNS and relay configuration are correct, and whether its IP addresses appear on any of 105 DNS-based blacklists. Together these checks help you decide whether a mail delivery problem comes from DNS configuration, server reachability, or blacklist status. You need the domain name you want to test; the lookup runs directly against that domain's authoritative name server, so recent MX record changes should appear immediately.

What the MX Lookup itself returns

The MX Lookup test lists the MX records for a domain in priority order. Because the query goes straight to the authoritative name server rather than a cached resolver, you are seeing the current published records, not a stale copy. This is the starting point for every other check, since Diagnostics and blacklist testing both operate on the mail servers those records point to.

What Diagnostics checks

After the MX records are listed, the Diagnostics option connects to the mail server and runs several checks in sequence:

  • Connection and response time — it contacts the mail server and measures how long it takes to respond, which tells you whether the host is reachable and performing normally.
  • Reverse DNS verification — it checks that the IP address of the mail server resolves back to a hostname. Missing or mismatched reverse DNS is a common reason receiving servers reject or distrust mail.
  • Simple Open Relay check — it tests whether the server will relay mail for an unrelated party. An open relay is a serious misconfiguration that spammers can exploit and that gets servers blocked.

Each result points to a different layer of the problem. A failed connection suggests reachability or firewall issues. A reverse DNS failure points to DNS configuration. An open relay finding is a server configuration problem you should fix before anything else.

What the blacklist check covers

You can check each MX record's IP address against 105 DNS-based blacklists, commonly called RBLs or DNSBLs. If a sending IP is listed, many receiving mail servers will reject or filter mail from it regardless of how correct the rest of your configuration is. This check is the fastest way to confirm or rule out blacklisting as the cause of delivery failures.

How to read the results together

Symptom Likely layer What to check first
Mail rejected or filtered Blacklist status Run the blacklist check on each MX IP
Mail rejected with DNS-related errors Reverse DNS Confirm the IP resolves back to the expected hostname
Server unreachable or slow Reachability / performance Review the Diagnostics connection and response time
Spam complaints or abuse reports Server configuration Review the Open Relay result

The practical value is narrowing the cause. If the MX records are correct and the server responds quickly but the IP is blacklisted, your problem is reputation, not DNS. If reverse DNS is missing, fixing that may resolve rejections even when the IP is clean. If the Open Relay check flags the server, treat it as a configuration issue to correct promptly.

Limits to keep in mind

These checks describe the current state of your mail server and its IPs at the moment you run them. They do not explain why a specific message was rejected by a specific recipient, and blacklist status can change as listings are added or removed. Use the results as a diagnostic starting point, then confirm the fix by re-running the same checks after you make changes.