AltF DomainOps

MX lookup: mail servers and email provider

An MX lookup lists the mail servers that accept email for a domain, lowest priority number first. DomainOps also names the provider when the hostnames match one (Google Workspace, Microsoft 365, Zoho, Proton and more) and flags a null MX, which means the domain deliberately accepts no mail.

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How MX priority works

Sending servers try the lowest number first and move to the next only if it fails. Equal numbers share the load. Priority values are relative: 1 and 5 behave the same as 10 and 50.

Common MX problems

  • MX points at a CNAME. RFC 2181 forbids it and some senders refuse to deliver.
  • MX points at an IP address instead of a hostname.
  • Old provider's MX left in place after a move, so some mail goes to a dead mailbox.
  • No MX at all, so senders fall back to the A record and mail usually bounces.

After the MX is right

Email reaching the right server is half the job. Receivers also check SPF, DKIM and DMARC before trusting a message; the Email DNS tab and the email domain setup guide cover those.

MX lookup: questions and answers

How do I find out who hosts a domain's email?

Run an MX lookup. The mail server hostnames usually reveal the provider, for example aspmx.l.google.com is Google Workspace and *.mail.protection.outlook.com is Microsoft 365. DomainOps names the provider when it recognises the hostname.

What is a null MX record?

A single MX record of "0 ." (RFC 7505). It tells senders the domain accepts no email, so they bounce immediately instead of retrying for days.

How long do MX changes take?

As long as the old MX record's TTL, often one hour. Senders that cached the old record keep using it until the TTL runs out.

Focused checks and guides

Checks

Full domain health checkAll nine tools, the record glossary and how each check works.DNS lookupFree DNS lookup for A, AAAA, CNAME, MX, NS, TXT, SOA, CAA, SRV, DS, DNSKEY and HTTPS records, with TTLs, DNSSEC status and what each record means.NS lookupFree nameserver lookup: see which nameservers answer for a domain, who the DNS host is, the SOA serial, and whether the zone is DNSSEC-signed.TXT record lookupFree TXT record lookup that labels each record: SPF, Google and Microsoft verification, Meta, Apple, Stripe and 25 more services, plus DMARC and DKIM.SSL certificate checkFree live SSL checker: connect to any site and see its certificate issuer, expiry date and days left, covered hostnames, chain trust and TLS version.Domain expiry checkCheck when a domain expires from the registry's own RDAP data: expiry date, days left, registrar, transfer lock and status codes in plain English.Reverse DNS lookupFree reverse DNS lookup for IPv4 and IPv6: find the PTR hostname of an IP address and the network owner from the regional internet registry.DNSSEC checkCheck whether a domain uses DNSSEC: DS record at the registry, DNSKEY in the zone and whether public resolvers validate the answers.CAA record checkLook up a domain's CAA records to see which certificate authorities may issue certificates, including wildcard and reporting rules, with fixes.

Guides

Domain launch checklistA 20-point checklist to launch a new domain: DNS records, www, HTTPS, email authentication, DNSSEC, CAA, registrar lock, with a free check for each step.Website migration DNS checklistMove a website or DNS to a new host without downtime: lower TTLs, copy records, switch nameservers, watch resolvers, handle DNSSEC and email.Email domain setup guideSet up email for your domain the right way: MX records, one SPF record under 10 lookups, DKIM keys, a DMARC rollout plan, MTA-STS and BIMI.

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AltFTool does not store the domains or addresses you check. DNS lookups in the DNS, compare, email and IP tools run from your browser; registration, report and certificate checks pass through AltFTool's server and keep nothing.