Do I need a backup MX record?

For most people, no. If your email runs on Google Workspace, Microsoft 365 or a similar hosted provider, redundancy is already built into the single MX record they give you. A self-managed backup MX mainly makes sense if you run your own mail server and want mail queued on hardware you control during a long outage.

What does a lower MX preference number mean?

It means higher priority. The MX record with the lowest number is your primary mail server. Microsoft's own setup docs state it directly: priority 0 is higher than priority 10.

What happens if my mail server goes down and I have no secondary MX?

Incoming mail is not lost right away. Sending servers queue the message and keep retrying for several days before bouncing it. If your server comes back within that window, the queued mail is delivered. A secondary MX only changes where the mail waits, not whether a short outage costs you mail.

Is a backup MX a security risk?

It can be if it is misconfigured. Backup MX servers are a frequent target for spam because they are often filtered less strictly than the primary. A backup that accepts mail it cannot deliver can also create backscatter. Run one only if you can filter it as tightly as your main server.

Can I list two email providers' MX records at once?

Generally, no. Pointing MX records at two different providers does not create a tidy backup; it splits delivery unpredictably and can break authentication. When moving between providers, cut the MX records over in one step rather than running both. Splitting mail across systems on purpose is a separate setup, split-domain routing, with its own rules.

Blog/ Email glossary & concepts

Email glossary & concepts

What Is a Secondary MX Record? Backup MX Myths Debunked

Nafiul HasanAugust 10, 2026· 7 min read

Diagram of a secondary MX record: DNS lists a primary mail server at a low preference number and a backup MX at a higher number that queues mail when the primary is down.

The short answer

A secondary MX record is a backup mail server listed in your domain's DNS with a higher preference number than the primary. Senders try it only when the primary is unreachable, so it queues mail during an outage. Most people on Google Workspace or Microsoft 365 do not need one, because the provider runs redundant servers behind a single record.

A secondary MX record is a backup mail server in your DNS. Learn how MX priority numbers work, what a backup MX really does, and whether you need one.

On this page

01What a secondary MX record is
02How do MX priority numbers work?
03What a backup MX does, and what happens without one
04Backup MX vs. provider-managed redundancy
05What most providers actually publish now
06Common myths about backup MX records
07A related signal: the Null MX record
08Where AI Emaily fits (and where it does not)

If you have opened your domain's DNS settings, you may have seen more than one MX record and wondered whether you need a secondary MX record as a backup. A secondary MX record is a second mail server your domain lists in case the first one cannot be reached. It sounds like insurance, and sometimes it is, but for most people on a modern email provider it solves a problem the provider has already solved.

This guide covers what a secondary MX record is, how MX priority numbers decide which server gets your mail, what a backup MX genuinely does, and why adding one does not improve deliverability. It also covers a real security risk that most articles skip.

What a secondary MX record is#

An MX (Mail eXchanger) record is a DNS entry that tells the rest of the internet which server accepts email for your domain. A domain can list several MX records. Each one carries a preference number, also called a priority, and that number sets the order in which senders try them.

A secondary MX record is simply any MX record with a higher preference number than your primary. It is the fallback. Sending servers only reach for it when the record with the lowest number cannot be reached. That is the entire job: hold mail while the main server is down.

How do MX priority numbers work?#

The rule catches people out because it feels backwards: the lower the preference number, the higher the priority. A server listed at 10 is tried before one listed at 20, and a record at 0 is tried before everything else.

Sending servers sort your MX records from the lowest number up and deliver to the first one that answers. If two records share the same number, the sender picks one at random to spread the load between them. If a domain has no MX record at all, the SMTP standard says to treat the domain's address (A/AAAA) record as an implicit mail server with a preference of 0.

Example MX records for one domain

example.com MX 1mail1.example.com (primary, tried first)

example.com MX 5mail2.example.com (secondary, the backup)

example.com MX 10mail3.example.com (last resort)

Lower number, higher priority

This ordering is set by the SMTP standard (RFC 5321) and echoed in provider docs. Microsoft's own setup page states it plainly: in MX records, lower numbers indicate higher priority, and priority 0 is higher than priority 10.

What a backup MX does, and what happens without one#

Picture your primary mail server going offline for an hour. Without a secondary MX, does your mail vanish? No. When a sending server cannot reach any of your listed mail servers, it does not bounce the message straight away. It queues the mail and keeps retrying for several days before it finally gives up.

So a short outage rarely loses mail even with a single MX record, because the sender's own queue is your safety net. A secondary MX mainly changes where the mail waits: on a server you control instead of on the sender's. That can matter if you run your own mail infrastructure and want copies held close, or you need mail flowing to a second site during a long outage.

The sender's queue is the real backup

For a domain on Google Workspace or Microsoft 365, the provider already runs many redundant servers behind one record. A do-it-yourself secondary MX mostly duplicates protection you already have.

Backup MX vs. provider-managed redundancy#

There are three common ways a domain handles mail-server redundancy. They are easy to confuse, and only one of them asks you to configure a separate backup.

Mail routing flow: a sending server tries the lowest-numbered MX first, and only falls back to a higher-numbered secondary MX when the primary does not answer. — Senders always try the lowest preference number first; the secondary MX is the fallback path.

Setup	How senders use it	Redundancy you get	Who runs it
Single provider-managed MX (for example smtp.google.com at priority 1, or an Outlook record at priority 0)	One hostname; the provider load-balances many servers behind it	High, built into the provider's network	Your email provider
Multiple equal-preference MX (same number)	Senders pick one at random, spreading load across them	High, traffic splits across the listed servers	Whoever runs the servers
Secondary / backup MX (higher number)	Used only when the lower-numbered server is unreachable	Queues mail during an outage of the primary	Usually you, on self-managed setups

What most providers actually publish now#

As of 2026, the large hosted providers lean on the first row of that table. Google Workspace tells admins to publish one record, smtp.google.com, at priority 1. Microsoft 365 publishes one record at priority 0. In both cases, redundancy lives inside the provider's network rather than in a second MX line you maintain.

This is a shift worth knowing. Older Google Workspace setups used five MX records with mixed priorities, so accounts created before 2023 may still show them, and plenty of old tutorials still teach that layout. Check your provider's current DNS page rather than a dated guide, because the record they recommend today may be a single line.

Common myths about backup MX records#

The biggest myth is that a secondary MX record helps your email reach the inbox. It does not. Deliverability is decided by authentication (SPF, DKIM, DMARC), your sending reputation and complaint rates, not by how many servers accept your mail. Adding a backup MX changes nothing a spam filter looks at.

A second myth is that more MX records always means more safety. In practice, a poorly run backup MX can add risk instead of removing it.

Why a lax backup MX is a target

Backup MX servers are a known target for spam, because they are often filtered more lightly than the primary, and mail relayed inward from a trusted backup can skip checks the primary would apply. A backup that accepts mail it cannot deliver can also generate backscatter. If you run one, filter it exactly as strictly as your primary.

MX records are how a domain states its mail intentions, and a backup is only one of those statements. The opposite statement also exists. A domain that accepts no mail at all can publish a Null MX record, a single record with preference 0 and a dot (".") as the server, to tell senders plainly not to try (RFC 7505).

A sender that hits a Null MX rejects the message rather than queuing it, using a 556 or 550 reply. It is the clean way to say a domain is not for email, and it is a useful contrast: a secondary MX invites a retry, a Null MX declines mail outright.

Where AI Emaily fits (and where it does not)#

A secondary MX record is a DNS and mail-server decision, and it belongs to whoever hosts your domain's email: Google Workspace, Microsoft 365, Fastmail or your own server. AI Emaily does not run MX records, host your mailbox or manage your DNS, and it cannot set up a backup MX for you. If that is the job you came to do, your provider's admin console is the place.

What AI Emaily does starts once mail lands in the mailbox you already have. It is an AI email client that connects to your Gmail, Outlook and IMAP accounts, then triages, drafts and files for you, with every send held for your approval and a full audit trail. The backup MX keeps mail arriving during an outage; AI Emaily handles the mail once it arrives. We build AI Emaily, and there is a 7-day free trial on the Pro and Autopilot plans, card required, $0 if you cancel before day 7.

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Keep reading

What Is an MX Record? How Email Finds Your Mail Server What Is a Mail Queue? Why Email Sometimes Sits Waiting What Is Split-Domain Email Routing and When Do You Need It?What Are Mail Routing Rules? Server-Side Mail Flow Explained

Sources

Written by

Nafiul Hasan

Nafiul Hasan is an entrepreneur and AI automation system builder with 10+ years of experience turning messy, manual workflows into reliable automated systems. He designs and ships AI enterprise solutions end-to-end — the agent logic, the data plumbing, and the product people actually use — and founded AI Emaily to give busy professionals their attention back. He writes here from the builder's seat: what works, what breaks, and how to put AI to work without giving up control.

EntrepreneurAI Automation System BuilderAI EnthusiastBuilds AI Enterprise Solutions10+ years experience