Glossary
Definition
An MX record(Mail Exchange record) is a DNS entry that names the mail servers responsible for receiving email for a domain. Each MX record carries a hostname and a priority value, and sending servers use the lowest priority first, falling back to the next, to learn where to deliver a domain’s mail.
We resolve MX records on every address we verify, and they are the quiet foundation the entire email system rests on. Before a message can travel, before a mailbox can even be checked, a sending server has to answer one question: where does this domain receive its mail? The MX record is the answer. Here is what an MX record actually is, how it routes mail, what happens when one is missing, and why we resolve it first.
MX stands for Mail Exchange. An MX record is a single line published in a domain’s DNS that names the hostname of a mail server willing to accept inbound email for that domain, paired with a number called its priority or preference. A domain almost never relies on just one. It publishes a set of MX records so that mail keeps flowing even when a server goes down.
The priority value is the whole trick. Lower numbers are tried first. A domain might publish mx1.company.com at priority 10 and mx2.company.com at priority 20. A sending server reads the list, sorts it, and attempts delivery to the lowest-priority host first. If mx1 is unreachable, it falls back to mx2, and so on down the list. When two records share the same priority, sending servers spread mail across them roughly evenly, which gives a domain both load balancing and redundancy from the same mechanism.
An MX record must name a hostname, never a bare IP address and never a CNAME alias. That named host then resolves to an IP through its own A or AAAA record. This is the detail that separates an MX record from the other DNS records people often confuse it with: an A record maps a name straight to an address, while an MX record adds a layer of indirection and a priority on top, specifically so a domain can move or duplicate its mail servers without changing where the world sends its mail.
Picture a message sent to you@company.com. The sending server splits the address at the @, takes the domain company.com, and queries DNS for its MX records. DNS hands back the ranked list of mail hosts. The sender opens a connection to the highest-priority host that answers and begins the SMTP conversation that actually transfers the message. The MX record is the map; SMTP is the journey that follows it.
This is also why the MX record is where deliverability begins. If the named host is healthy and accepts the connection, mail has a path. If every host on the list refuses or times out, delivery stalls and the sender retries on a schedule before eventually bouncing. The MX record does not guarantee a specific mailbox exists, but it does guarantee there is a server prepared to receive mail for the domain at all, which is the first thing any verification has to establish.
When a domain publishes no MX record, the mail standards allow a sending server to fall back to the domain’s A record and try delivering there instead. In theory that is a graceful degradation. In practice it almost never works, because the host on the A record is usually a web server with no mailbox listening, so the message bounces.
So a missing MX record is one of the clearest deliverability signals there is: it strongly implies the domain cannot receive email. Parked domains, typo domains, and abandoned domains frequently have no MX at all. Treating an address on such a domain as deliverable is how an otherwise clean list still posts a bounce rate that damages sender reputation. A domain with valid MX records is not a promise the specific mailbox is real, but a domain with no MX record is very nearly a promise that it is not.
MX resolution is the first of the nine checks our engine runs on every address, the same engine behind the free email checker and the paid API. Before any SMTP handshake, the engine takes the domain, queries its MX records, and sorts them by priority. That one step decides everything downstream.
If the domain returns no MX record, the address fails fast: the engine never opens an SMTP connection, because there is no server to connect to. That saves a wasted round trip and returns a clear, immediate verdict instead of a slow timeout. When MX records do exist, the engine selects the highest-priority host that answers and hands it to the SMTP stage, which then probes whether the specific mailbox is real. In other words, the MX record tells the engine where to ask, and SMTP asks the question.
Resolving MX first is what makes the rest of the pipeline both fast and accurate. You can look up any domain’s MX records on their own with our MX lookup, see the surrounding mail configuration with the mail domain lookup, and confirm a named server actually answers with the SMTP test. For teams verifying at scale, the volume tiers are on the pricing page.
Keep learning
The MX record sits at the start of a wider set of deliverability concepts. These are the terms most worth understanding next.
The protocol mail servers speak to hand a message from sender to recipient.
A DNS record listing which servers are allowed to send mail for a domain.
A cryptographic signature that proves a message was not altered in transit.
Confirming an address is real, reachable, and safe to send to before you send.
A permanent delivery failure, often because the domain cannot receive mail.
A domain that accepts mail for any username, valid mailbox or not.
MX resolution is the first thing our engine does. Enter a domain in the MX lookup and see every mail server and priority, in order, in seconds. No signup.
Common questions
The questions we get most about MX records, answered with the same logic our verification engine uses.
It is the line in a domain’s DNS that says “send this domain’s email here.” When someone mails you@company.com, the sending server looks up the MX record for company.com to find which mail server should receive the message.
Without that signpost, a sending server has no idea where a domain’s inbox lives, so the message has nowhere to go.
MX stands for Mail Exchange(sometimes written Mail Exchanger). The record points at the mail exchanger, the server that accepts inbound email on the domain’s behalf, which is why a single domain can list several MX records for redundancy.
Each MX record has a priority number, also called a preference. Lower numbers win. A sending server tries the MX with the lowest priority first; if that host is unreachable, it falls back to the next lowest, and so on down the list.
When two records share the same priority, mail is balanced across them roughly evenly, which is how providers spread load and add redundancy at once.
Yes, and most production domains do. A primary mail server gets a low priority, and one or more backups get higher numbers so they only take over if the primary is down. Equal priorities share the inbound load between servers.
You can see the full set for any domain with our MX lookup, which lists every host and its priority in order.
By the mail standards, a sending server may fall back to the domain’s A record and try delivering to that address instead. In practice that fallback rarely works, because the host on the A record is usually a web server, not a mailbox, so the mail bounces.
A missing MX record almost always means the domain cannot receive email, which is why we treat no MX as a strong deliverability red flag during verification.
An A record maps a hostname to an IP address, and a CNAME aliases one hostname to another. An MX record does neither directly: it names the hostname of a mail server and gives it a priority. That mail host then has its own A record resolving to an IP.
One rule worth knowing: an MX record must point at a real hostname, never directly at an IP and never at a CNAME.
MX resolution is the first of the nine checks our engine runs. Before any SMTPhandshake, the engine resolves the domain’s MX records, picks the highest-priority host, and opens the connection there.
If there is no MX record at all, the address fails fast and never reaches the SMTP stage, which saves a round trip and gives you a clear verdict. You can watch it run on the free email checker.
The fastest way is our MX lookup: enter a domain and it returns every MX host and priority in seconds. For a full picture of a domain’s mail setup, the mail domain lookup adds the surrounding records, and the SMTP test confirms the named server actually answers.