# How to mark a mail domain as backup MX

This tutorial describes how to mark a mail domain as a backup MX.

## How backup MX works in brief

When smtp service on primary MX server is down, emails will be delivered to backup MX server. When backup MX server detects smtp service on primary MX is back online, it will relay received emails to primary MX.

## Make sure you have correct DNS records for the mail domain

To let other mail servers know your server is the backup MX server of this mail domain, you must add your mail server info in its MX type DNS record.

For example, main mail server of your mail domain `example.com` is `mx01.example.com`, to set your iRedMail server `mx02.example.com` as backup MX of `example.com`, you need to add `mx02.example.com` as your lower priority mail server in MX type DNS record.

    example.com.    3600    IN  MX  5   mx01.example.com
    example.com.    3600    IN  MX  10  mx02.example.com

Server `mx01.example.com` has priority number `5`, and `mx02.example.com` has priority number `10`. For mail service, the lowest number has highest priority. so with above example, server `mx01.example.com` is the primary MX, and `mx02.example.com` is a backup MX.

When mail service on `mx01.example.com` is down, other mail services will connect to `mx02.example.com`.

## Mark domain as backup MX with iRedAdmin-Pro

It's easy to manage domain profile With iRedAdmin-Pro, including backup MX setting. Go to domain profile page, click tab `Backup MX`:

## Mark domain as backup MX with command line tools

### For SQL backends

Primary server address

It's recommended to set the IP address of primary MX as relay server to avoid mail loop.

SQL commands (we use MySQL for example):

    USE vmail;
    UPDATE domain SET transport='relay:[45.56.127.226]:25',backupmx=1 WHERE domain='example.com';

In above example, we mark domain `example.com` as a backup MX, and use IP address `45.56.127.226` as primary MX server, you should replace it by the real IP address.

### For LDAP backends

In domain object, please add LDAP attribute/value pair like below:

    domainBackupMX: yes
    mtaTransport: relay:[45.56.127.226]:25

## See also

- Backup MX (wikipedia)