Bulk MX redundancy checker: which MX is primary, which is backup, and do they work?
In short: Paste domains and see each one's MX records labelled primary, shared primary or backup, with every host checked for an address and for the alias and IP-address mistakes. Limit: Only DNS is checked. No mail server is contacted, so a host with an address can still be down.
A bulk MX redundancy checker explains the MX records of many domains in plain terms. Paste up to 100 domains, or 500 after a free Google sign-in, and each row labels the lowest preference number as primary, equal numbers as shared and higher ones as backup, then checks each host has an address and isn't an alias or an IP. DNS only. Download the CSV.
Price: this tool is free. Checking a whole email list is from $9/mo, and the first 100 checks are free with Google, no card. Check your list free nowSee pricing
Sample result
acme.example has a primary and a backup MX; globex.example's only MX host is an alias.
- Domain
- acme.example
- 10
- mx1.acme.example, primary
- 20
- mx2.acme.example, backup
What this means for you
An MX target must not be a CNAME, and some senders refuse to deliver to one.
- Primary, shared and backup roles
- Alias and IP-address targets flagged
- Hosts with no address flagged
- Null MX recognised; CSV out
- MX
- Backup MX
- Priority
By SimpleVerifier Team · Last updated
Auditing the email DNS of a domain list?
- Paste the domains here: up to 100 a run, or 500 after a free Google sign-in. Filter to the rows that need a fix and download the CSV.
- Upload or paste it at /verify. Every address gets a live mailbox check, and the first 100 email checks are free with a Google sign-in, no card.
How do I check the MX priorities of many domains?
- Paste domains, email addresses or URLs, one per line, up to 100 a run or 500 after a free Google sign-in.
- Press Explain MX priorities. Each domain's MX records are read, then the A, AAAA and CNAME records of each MX host, 10 domains a request. No connection is made to the mail servers.
- Read each row: More than one MX host, One MX host, MX problem (the fault is shown), Null MX, No MX records or Lookup failed, with each host's role.
- Filter to the MX problems and download the CSV.
Example
- You enter
- Three domains:
acme.example(MX 10 and MX 20),globex.example(one MX that is a CNAME) andinitech.example(a null MX). - You get
- acme.example: More than one MX host, the 10 host labelled primary and the 20 host backup. globex.example: MX problem, the host is an alias. initech.example: Null MX, takes no mail. Download the CSV.
- Limits
- Up to 100 a run without an account and 500 when signed in with Google, counted over a minute. 10 domains a request; DNS only, up to 10 MX records each.
How do MX priorities work?
Each MX record has a preference number and a host. Senders try the host with the lowest number first and move to a higher number only if they can't reach it. Hosts with the same number share the load, and senders pick between them. RFC 5321 section 5.1 describes the order.
So a record set of 10 mx1, 20 mx2 has a primary and a backup, while 10 mx1, 10 mx2 has two equal servers. The numbers only matter relative to each other; 1 and 2 behave exactly like 10 and 20.
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 don't, which makes them an easy way around filtering and a source of backscatter. Hosted providers such as Google Workspace and Microsoft 365 publish several equal or tiered hosts for you, and they don't need an extra backup.
Senders also retry for days when a primary is down, so a short outage rarely loses mail. This tool doesn't judge whether a backup is wise; it shows what you have and whether each host works.
What counts as an MX problem here?
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 can't connect to it. A null MX (0 .) and a missing MX are reported separately; see the null MX explainer.
One MX host is reported as such and isn't a fault: a hosted provider can run many servers behind one name. To list MX records and the mail provider behind them, use the bulk MX lookup.
Who uses this tool, and for what?
- Email administrators
- Check a domain's MX set after a provider migration or a DNS edit.
- MSPs
- Audit client domains for aliases, IP targets and hosts that no longer resolve.
- Deliverability consultants
- Add an MX health column to a domain audit and export it as CSV.
- Sales and marketing teams
- Spot domains in a list whose MX is broken before you send to them.
What can’t this tool tell you?
- Only DNS is checked. No mail server is contacted, so a host with an address can still be down.
- At most the first 10 MX records of a domain are read.
- It doesn't judge whether a backup MX is a good idea, only what is published.
- Up to 100 domains a run without an account, 500 signed in.
Common questions
- How do I check the MX priorities of many domains?
- Paste the domains into a bulk MX redundancy checker like this one. It reads each MX set, labels primary and backup hosts and checks every host, with a CSV of the results.
- Is this bulk MX redundancy checker free?
- Yes. It is a free tool: up to 100 domains a run with no account, and up to 500 a run when you sign in with Google, with no card.
- Which MX record is tried first?
- The one with the lowest preference number. Equal numbers share the load.
- Can an MX record point to a CNAME?
- It shouldn't. RFC 2181 says an MX target must not be an alias, and some senders refuse to deliver to one.
- Can an MX record point to an IP address?
- No. An MX record names a host. An IP written in the host field is a mistake that senders can't follow.
- Why does a big provider show a single MX host?
- Some providers publish one name that resolves to many servers. One host isn't a fault in itself.
Sources
A tidy list can still be full of dead addresses.
Extracting, deduplicating and syntax checks all work on the text of an address. None of them can tell whether the mailbox exists. Only the server that hosts it can answer that, through an SMTP check, and that is what the paid product does.
- Upload the cleaned file as it is: CSVs up to 100,000 rows
- A real SMTP mailbox check on every row, with catch-all detection
- Small list? Starter at $9/mo covers 2,500 emails a month, cancel anytime
Plans from $9 a month, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.