How to Tell Which MX Record Is Primary and Which Is Backup (And Whether They Work)

To tell which MX record is primary, look at the preference numbers: the lowest is tried first, equal numbers share the load, and higher numbers are backups. The bulk MX redundancy checker labels each host that way for up to 100 domains a run, 500 after a free Google sign-in, then checks each host has an address and is not an alias or an IP. No mail server is contacted.

How MX priorities work

Each MX record has a preference number and a host. Senders try the lowest number first and move to a higher one only if they cannot reach it. RFC 5321 section 5.1 describes the order. So 10 mx1 and 20 mx2 give a primary and a backup, while 10 mx1 and 10 mx2 give two equal servers. Only the relative order matters.

Doing it by hand

dig +short MX example.com
dig +short A mx1.example.com
dig +short AAAA mx1.example.com
dig +short CNAME mx1.example.com

The first lists the records, each as a number and a host. For each host, the next three should show an A or AAAA answer and no CNAME. Repeat for every domain.

Step 1: paste the domains

Paste domains, email addresses or URLs, one per line, up to 100 a run or 500 after a free Google sign-in.

Step 2: run the check

Each domain's MX records are read, then the A, AAAA and CNAME records of each MX host, 10 domains a request.

Step 3: read each row

Result	What it means	What to do
More than one MX host	Each host is labelled primary, shared or backup	Nothing to fix
One MX host	A single host	Not a fault; a hosted provider may run many servers behind it
MX problem	The fault is shown	Fix the host or the record
Null MX	The domain says it receives no mail	See the null MX explainer
No MX records	Nothing published	Add MX records if the domain should receive mail
Lookup failed	DNS did not answer properly	Try again later

What counts as an MX problem

Three mistakes are flagged: an MX host that is an IP address instead of a hostname, an MX host that is an alias (a CNAME), which RFC 2181 section 10.3 forbids, and an MX host with no A or AAAA record, so senders cannot connect to it. For more on these, read MX record errors explained.

Is a backup MX a good idea

It depends. A backup that holds mail while the primary is down only helps if it applies the same spam and mailbox checks as the primary. Many backups do not, which makes them an easy way around filtering. Hosted providers publish several hosts for you and do not need an extra backup. Senders also retry for days when a primary is down, so a short outage rarely loses mail. The tool shows what you have and whether each host works; it does not judge whether a backup is wise.

What this check cannot do

• It checks DNS only. A host with an address can still be down.
• At most the first 10 MX records of a domain are read.
• The limit is 100 domains a run without an account and 500 signed in.

To list MX records and the mail provider behind them, use the bulk MX lookup. For Google Workspace specifically, see Google Workspace MX records.

Check the addresses you send to

Correct DNS gets mail to the door; the address still has to exist. Upload your list on the verify page: the first 100 email checks are free with a Google sign-in, no card. Each address comes back valid, risky or invalid with a reason, and nothing is sent to anyone on the list. For larger or recurring lists, see bulk email verification, list cleaning and pricing.

Check your domains now

Paste them into the bulk MX redundancy checker.