An MX record is the DNS record that routes email to a domain's mail server. This guide explains the syntax, how priority orders delivery, how to look records up, and why an MX record alone cannot tell you whether a domain is a catch-all.
An MX record (Mail Exchange record) is a DNS resource record that specifies which mail server is responsible for accepting email on behalf of a domain. Each record pairs a server hostname with a priority number - lower numbers are tried first.
When you send an email, your mail server must figure out where to deliver it. It does three lookups:
The sending server queries DNS for the recipient domain's MX records to find the mail servers.
It sorts the returned records by preference number. The lowest number is the primary server; higher numbers are fallbacks for when it is down.
It connects to the chosen server on port 25 and delivers the message via SMTP.
Without an MX record, most mail servers reject email to the domain with a permanent error (often 550 No MX record), so the message cannot be delivered at all.
A complete MX record has three parts: the preference number, the mail server hostname, and the TTL.
A catch-all domain accepts email to any address at that domain - even ones that don't exist. This behaviour is configured on the mail server itself, not in DNS. A normal domain and a catch-all domain publish identical MX records pointing at the same server; only the server's response to a RCPT TO probe reveals the difference. That is why catch-all detection requires an SMTP probe, never a DNS lookup.
BounceZero reads MX records in the first stage of verification to confirm the domain can receive email at all, identify the mail provider (Gmail, Outlook, Google Workspace, a custom server), and then routes the SMTP probe accordingly. The MX record selects the server; the probe determines the verdict.
No terminal handy? Use the free BounceZero MX Lookup tool - it returns a domain's mail servers and whether they accept mail for the domain.
An MX (Mail Exchange) record is a DNS record that tells other mail servers which host is responsible for receiving email for a domain, along with a priority value. Without a valid MX record, a domain cannot receive email.
The priority (preference) number orders the servers. Lower numbers are preferred, so senders try the lowest first and only fall back to higher numbers when the preferred server is unreachable. Two records can share the same priority for load balancing.
No. Catch-all behaviour is a server-side configuration that returns 250 OK to every RCPT TO probe. Normal and catch-all domains publish identical MX records, so detection requires actually probing the server, not reading DNS.
Run `dig MX example.com` or `nslookup -type=MX example.com`, or use a free online MX lookup. BounceZero's MX Lookup tool shows a domain's mail servers and whether they accept mail.
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