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MX records are DNS entries that tell the internet which mail servers accept email for a domain, and in what order. You look them up to confirm a domain is actually configured to receive mail and to diagnose why messages bounce or disappear. If email to your domain fails, an MX lookup is usually the first check.

What an MX record actually contains

An MX (Mail Exchange) record maps a domain to one or more mail server hostnames. Each record carries a priority number, and the sending server tries the lowest number first. If that server is unreachable, it moves to the next priority.

A typical set looks like this:

The priority value is relative, not absolute — only the ordering matters. A domain with a single MX record has no fallback, so if that server goes down, mail is delayed or bounced.

Why you should look them up

• Confirm mail is configured at all. A domain with no MX record cannot receive email, even if it has a website.
• Troubleshoot delivery failures. If senders report bounces, a wrong or missing MX record is a common cause.
• Verify a migration. After moving mail to a new provider, the MX records must point to the new servers.
• Check for stale or conflicting entries. Old records left behind can route mail to servers that no longer exist.

How to run an MX lookup on MxToolbox

MxToolbox provides a free MX Lookup tool at mxtoolbox.com. The page states the test lists MX records for a domain in priority order.

1. Enter the domain name you want to check.
2. Run the lookup. The tool queries the domain's authoritative name server directly, so recent MX changes should appear immediately rather than waiting for cached copies to expire.
3. Read the results as a priority-ordered list of mail servers.

From the results you can go further:

• Diagnostics connects to the mail server, verifies reverse DNS records, performs a simple Open Relay check, and measures response time.
• Blacklist check tests each MX record's IP address against 105 DNS-based blacklists (RBLs/DNSBLs).

Note that the site requires JavaScript to run, so the lookup will not work with scripting disabled.

What to look for in the results

• No records returned — the domain is not set up to receive mail. This is the most common reason a domain silently drops email.
• Records pointing to an unexpected provider — mail is being delivered somewhere you did not intend, often a leftover from a previous host.
• A single record with no backup — no redundancy if that server fails.
• Priority values that contradict your intended order — the lowest number wins, so a mislabeled backup can become the primary path.
• Server unreachable or failing diagnostics — the record exists but the server does not respond, which points to a hosting or firewall problem rather than a DNS one.

Where this fits in a broader check

MX records only cover inbound mail routing. They say nothing about SPF, DKIM, or DMARC, which govern whether your outgoing mail is trusted. If mail is being rejected as spam rather than failing to arrive, the MX lookup will look correct and the problem lies elsewhere. Use the MX lookup first when the question is "is this domain set up to receive mail, and where does it go?"

MxToolbox is a free online network diagnostics platform best known for its MX Lookup tool, which lists a domain's mail exchanger (MX) records in priority order. It's aimed at IT staff, email administrators, and domain owners who need to troubleshoot email delivery problems or verify DNS configuration. The MX lookup queries the domain's authoritative name server directly, so record changes should appear immediately rather than waiting on cached results.

What the MX Lookup tool does

The core function is straightforward: you enter a domain name, and the tool returns its MX records sorted by priority. Because the query goes straight to the authoritative name server, you see the current state of the zone rather than a stale resolver cache.

From there, the tool offers additional checks:

• Diagnostics — connects to the mail server, verifies reverse DNS records, runs a simple Open Relay check, and measures response time performance.
• Blacklist checks — each MX record (by IP address) can be checked against 105 DNS-based blacklists, commonly called RBLs or DNSBLs.

These checks matter because MX records alone don't tell you whether mail will actually be delivered. A correctly configured MX record pointing at an IP that appears on a blacklist, or one with a broken reverse DNS entry, can still cause delivery failures.

Who it's for

The tool fits a few common situations:

• Email administrators diagnosing why messages to or from a domain are bouncing or being rejected.
• IT staff verifying that DNS changes propagated as expected after a migration or provider switch.
• Domain owners checking whether their mail setup is configured correctly, especially after changing hosting or email providers.

Practical notes

The site requires JavaScript to function, so a browser with JavaScript enabled is necessary. The MX lookup itself needs no account or login based on the available information. Beyond the lookup, the diagnostics and blacklist checks are the features most likely to surface the actual cause of a delivery problem — a working MX record is only the first step.

For a quick answer to "is this domain's mail routing set up correctly?", the MX Lookup is the starting point. For "why is mail from this domain being blocked?", the diagnostics and blacklist checks are where the useful detail lives.

Yes — MxToolbox lists an API entry point on its site, so an API exists as part of the service. The MX Lookup page itself does not spell out API endpoints, authentication methods, rate limits, or pricing, so those specifics have to be confirmed through the API link or by contacting MxToolbox directly. What the page does make clear is a set of baseline conditions for using the service at all: JavaScript must be enabled, and use is governed by the site's Terms & Conditions and Privacy Policy.

What the MX Lookup page confirms

The page evidence for the MX Lookup tool shows the following, which is the extent of what can be stated from the source:

Because the page only surfaces the API as a navigation link, treat it as a pointer rather than documentation. Anything about request formats, keys, quotas, or commercial terms is not established by this page.

Conditions you can rely on before using the service

JavaScript must be enabled

The page explicitly states that JavaScript is required and that it is currently disabled. In practice this means:

• Input: you load the MX Lookup page in a browser with JavaScript turned off.
• Action: the page cannot run its lookup interface.
• Expected result: you see the JavaScript-disabled notice instead of a working tool.

If you are scripting or automating lookups, this is a signal that the browser-facing tool is not designed to work without JavaScript — a separate API path, if available, would be the relevant route rather than scraping the page.

Terms and privacy apply

Use of the service is subject to the linked Terms & Conditions and Privacy Policy. Read both before relying on the service for anything beyond casual checks, particularly if you plan to query domains you do not own or to feed results into another system.

Contact channel

The page lists a phone number, (866)-698-6652, as a contact route. This is the only contact method shown in the page evidence; no email address or support form is given there.

How to actually confirm API details

Since the page does not document the API, work through these steps:

1. Open the API link in the site footer. That is the entry point the site itself provides.
2. Check what the API page states about access — whether it requires an account, a key, or a paid plan. The MX Lookup page gives no pricing signals, so do not assume the API is free or open.
3. Read Terms & Conditions and Privacy before sending automated traffic, so you know what usage is permitted.
4. If the API page is unclear, use the listed phone number to ask about access requirements and limits.

What the MX Lookup tool itself does

Understanding the tool helps you judge whether you need the API at all. Per the page:

• It lists MX records for a domain in priority order.
• The lookup runs directly against the domain's authoritative name server, so MX record changes should show up instantly.
• A Diagnostics option connects to the mail server, verifies reverse DNS records, performs a simple Open Relay check, and measures response time.
• Each MX record (IP address) can be checked against 105 DNS-based blacklists (RBLs / DNSBLs).

If your need is occasional manual checking, the web tool covers it. If you need repeatable, programmatic checks, the API is the relevant path — but its requirements are not stated on this page and must be verified at the source.

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

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.

Go to mxtoolbox.com's MX Lookup page, enter the domain you want to check, and submit it. The tool returns that domain's MX records in priority order. Because the lookup runs directly against the domain's authoritative name server, recent MX record changes should appear immediately rather than waiting on cached results. From the results you can also run Diagnostics on a mail server, which connects to it, verifies reverse DNS records, performs a simple Open Relay check, and measures response time.

What the MX Lookup tool does

MX records tell the internet which mail servers accept email for a domain, and in what order they should be tried. MxToolbox's MX Lookup lists those records for a domain in priority order.

The page's own description notes one detail that matters when you are troubleshooting: the lookup is done directly against the domain's authoritative name server, so changes to MX records should show up instantly. That makes it useful right after you edit DNS and want to confirm the change is live.

Step-by-step: running a lookup

1. Open the MX Lookup tool on mxtoolbox.com.
2. Enter the domain name you want to check (for example, the domain whose email delivery you are diagnosing).
3. Submit the lookup.
4. Read the results: MX records are listed in priority order, so you can see which mail server is preferred and which are fallbacks.

Note that the site requires JavaScript — the page states JavaScript is required for the site, so a browser with JavaScript disabled will not run the tool.

Going further with Diagnostics

Once you have the MX records, you can click Diagnostics. According to the tool's description, Diagnostics will:

• Connect to the mail server
• Verify reverse DNS records
• Perform a simple Open Relay check
• Measure response time performance

You can also check each MX record (its IP address) against 105 DNS-based blacklists, commonly called RBLs or DNSBLs.

What you can conclude from the results

Practical notes

• The tool reports your own IP on the page, which is handy context when you are checking whether your sending server is the one being tested.
• Blacklist checks are per MX record IP, so a domain with several mail servers may show different results for each.
• The page lists contact and legal links (Terms & Conditions, Security, API, Privacy) plus a phone number, so there is an API path if you want to automate lookups rather than use the web form.