The MX record tells the rest of the internet which servers receive email for your domain.
Key points
- MX records were defined in RFC 1035; each one holds a preference value and a host name, and lower numbers are tried first [1].
- Under SMTP, a sender looks up the recipient domain's MX records and falls back to the domain's own address only if none exist [2].
- A domain that never receives mail can publish a null MX, a single record with preference 0 and a target of a dot, defined in RFC 7505 [3].
- Google Workspace domains point MX to smtp.google.com, which replaced an older five-record setup [4].
- MX records control inbound mail only; outbound authentication relies on SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail) and DMARC, which are separate DNS Record types.
- Email Verification tools check for valid MX records before sending, since a domain with none cannot receive mail and will cause a Hard Bounce.
How MX records route email
When a server needs to deliver a message to someone at example.com, it queries DNS for the MX records of example.com. Each record lists a mail server host name and a preference number [1]. The sender tries the host with the lowest number first and moves to higher numbers if that server is unreachable, which is how organizations build backup mail routes. If a domain has no MX record at all, RFC 5321 says the sender should treat the domain's own A or AAAA address as an implicit MX [2]. The MX target must be a host name with its own address records, not an IP address or an alias. Getting these details wrong usually shows up as bounced or delayed inbound mail rather than a problem with sending.
MX records and outbound email
MX records say where mail for a domain should arrive, not which servers may send it. Still, they matter to senders in two ways. First, receivers may be suspicious of mail from a domain that cannot accept replies, since legitimate senders expect responses and bounces. A Cold Email domain with no working MX looks disposable. Second, list quality checks rely on them: an Email Verification service looks up the MX of each prospect's domain and can flag domains with none, which prevents bounces that would raise the Bounce Rate. Domains that exist only for branding or parked pages should publish a null MX so that senders fail fast instead of retrying [3]. Pairing that with a reject-all SPF (Sender Policy Framework) record and DMARC policy blocks spoofing too.
Setting up MX records
MX records are managed wherever the domain's DNS is hosted, usually the registrar or a DNS provider. Mailbox providers publish the exact values to use. Google Workspace, for example, now asks for a single MX record pointing to smtp.google.com with priority 1, replacing the older set of five aspmx hosts [4]. Changes can take up to the record's TTL to spread, so teams moving providers often lower the TTL a day in advance. When setting up a Secondary Sending Domain for outreach, give it working MX records so that replies reach a real mailbox. Replies are the goal of outreach, and a reply that bounces is a lost conversation. After any change, a quick DNS lookup or test message confirms that mail flows in both directions.
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