7.10 Setting up the backup mail server
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## 7.10 Setting up the backup mail server

### Information and Requirements

1. A company has own `company.com` domain, the primary MX record points to the computer where primary mailserver is installed. The primary mail server's DNS name is `mail1.company.com`.

2. Create the backup server for the primary mailserver (its DNS name will be `mail2.company.com`). A basic version of Kerio MailServer can be used, because in this case there is no need to create user accounts.

### Implementation

1. Create the secondary MX record (with lower priority) in DNS for the `company.com` mail domain for (`mail2.company.com`) backup server.

2. After the backup of Kerio MailServer is installed, create a primary domain in the configuration wizard and assign it the same name as the primary mailserver, i.e. `company.com`.

3. No user accounts are set up in this domain.

4. In Configuration → Domains section of the Kerio MailServer administration console, specify message forwarding to the `mail1.company.com` primary mailserver (see picture 7.13 Setting up the backup server — the ETRN command).

   There are multiple ways of forwarding messages:

   - The best way of setting up forwarding from the backup server is to set the primary server in the way that it queries the secondary server regularly using the ETRN command. This procedure saves time because the servers are not connected to an unavailable primary server. The primary server must support the ETRN command.

     Kerio MailServer supports using the ETRN command for requesting emails (see chapter 12.5 Receiving Email Using ETRN Command). If you use Kerio MailServer as a primary mailserver, we recommend this option. Kerio MailServer also sends the ETRN command to different servers upon each server startup and thus all mail is downloaded to the server in the shortest possible time after failure.

     If you want to use this method of email forwarding, allow the Offline delivery — delivery is triggered by ETRN command from remote host option (see figure 7.13 Setting up the backup server — the ETRN command) the `company.com` domain on the backup server in the administration console (Configuration → Domains).

     Figure 7.13. Setting up the backup server — the ETRN command

     It is also necessary to enable using the ETRN command in the primary mailserver (see chapter 12.5 Receiving Email Using ETRN Command) and schedule sending the ETRN command (see chapter 20 Scheduling).

   - Another possibility is setting up the rules for outgoing messages (see chapter 12.2 SMTP server). However, in case of unavailability of the primary server, the server will repeatedly attempt to deliver emails, until the primary server is up and running again, which can occasionally cause overloading of the primary server.

     If you prefer this method of setting the secondary SMTP server, we recommend to extend the interval for message resending. This can be set in Configuration → SMTP Server, on the Queue Options tab.

     Figure 7.14. Setting up the backup server — mail delivery follows rules for queue of outgoing messages

     In the domain configuration window, it is necessary to set name or address of the primary server, traffic port and the Online delivery — deliver the messages immediately (see figure 7.14 Setting up the backup server — mail delivery follows rules for queue of outgoing messages).

   - The last method is to set up the scheduler so that it adjusts the intervals for sending emails. This setting is similar to the previous one, because the server again uses the rules for the outgoing message queue. However, in this case, the interval is adjusted by a scheduler, where more convenient schedule can be set.

     In the Configuration → Domains menu, the Forwarding tab of the domain company.com, you must enable the option The forward host is offline, delivery is triggered by scheduler (for details on scheduler's settings, refer to chapter 20 Scheduling).

     Figure 7.15. Setting up the backup server — mail delivery is controlled by the scheduler

5. If Kerio MailServer is used as a primary mailserver, we recommend to add the server address to the list of ignored servers that are not restricted by the settings in the Configuration → SMTP server menu of the Security options tab (for more information, see chapter 12.2 SMTP server).

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