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Backup MX: what a fallback mail server is for, and do you need one?

· Published on 02/10/2026 · 6 min read

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If you run your own mail server, an outage, a network cut or maintenance that takes longer than planned will happen sooner or later. The question is then simple: what happens to the emails your customers send you in the meantime?

A backup MX, or secondary MX, is one answer to that problem. This guide explains how it works, what it really brings and when it is worth having.

What happens when your mail server cannot be reached?

When someone writes to you, their sending server looks up the MX records of your domain name in the DNS. These records say which server receives mail for your domain. If that server does not answer, the sender does not lose the message straight away: it keeps it in a queue and retries regularly.

The SMTP standard recommends retrying for at least 4 to 5 days. In practice, every provider does as it sees fit: some retry for several days, others for much less. When the delay runs out, the message is rejected and the sender receives a delivery failure notice.

During the outage you see nothing arrive, you do not know who tried to reach you, and some messages can be lost if the outage lasts too long. For a business, a quote request or an order that never arrives is a lost customer.

How a backup MX works

A domain name can have several MX records, each with a priority. The lowest number is the preferred server. For example:

  • MX 10 mail.your-company.be: your main server
  • MX 20 fallback server: the backup MX

Normally, senders deliver to the priority 10 server. If it does not answer, they move on to the next one. The backup MX then accepts the message, keeps it, and hands it over to your main server as soon as it is reachable again. For the people writing to you, everything works as if your server had stayed online.

A backup MX does not replace your mail server: it is not an inbox. It holds mail and forwards it, nothing more.

Points to watch

A badly configured backup MX can do more harm than good. Before choosing one, check these points:

  • Spam filtering: spammers often target secondary MX servers, because they are sometimes less well protected than the main server. The backup MX must filter spam and viruses as well as your own server does, and check SPF and DKIM.
  • Confidentiality: your messages are stored by a third party during the outage. They must be stored encrypted, and deleted as soon as they have been delivered.
  • Retention period: a backup MX must be able to keep mail long enough to cover a real outage, without keeping it forever.
  • Visibility: it helps to be able to see what is waiting, rather than waiting without knowing.
  • It is not a backup of your mailboxes: if you lose emails already received on your server, the backup MX will not give them back. Keep real backups as well.

Who really needs one?

A backup MX is useful if you host your own email: a server at the office, a VPS, or a solution such as Mailcow or Mail-in-a-Box. This is all the more true if email is an important channel for your business: orders, quote requests, exchanges with suppliers.

On the other hand, if your email is with a large provider such as Microsoft 365 or Google Workspace, a backup MX is usually unnecessary: their infrastructure is already redundant.

Setting it up

Configuration happens in the DNS of your domain. With the Xeron Backup MX, two records are enough:

  • a TXT record, to prove that the domain belongs to you;
  • an MX record pointing to the fallback server, with a higher number than your current MX (for example 20 if yours is 10).

Your main server keeps receiving mail as usual. The backup MX only steps in when it cannot be reached, then automatically delivers all the mail it held. The service runs on XeronMX, open-source software whose code is public.