These days, almost all business communications happen over email. If the mail server goes down for any reason, all mails in that server will bounce, causing significant business disruption.

For shared hosting providers, this is a major issue, as a single server hosts hundreds of businesses, and a service failure can affect all of them. During such a downtime, hosting providers are usually flooded with support requests to recover lost emails.

Here at Bobcares, we help hosting companies manage their support desk (via our Outsourced Technical Support services), and we’ve seen our fair share of support requests complaining about lost mail. Broadly there are two categories of such issues:

• Delayed mail delivery : When the retry time is high for sending mail server. Mails will eventually be delivered, but will be delayed by a few hours to even a day.
• Permanent bounce : When the retry time-out has expired or there are no retry attempts, causing mail loss during a server down.

Neither of these two situations are desirable. Many mails would be of urgent nature, and hosting users won’t have the patience to wait for it to be eventually delivered. A permanent bounce is even worse, because any information sent is permanently lost.

To counter these situations, we recommend hosting companies to setup an email bagging server using a backup mail server (aka backup MX).

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What is a Backup MX?

Every domain that requires email services, has an associated MX record in its DNS zone. This MX (Mail exchanger) record specifies the mail server responsible for handling the mails of that domain.

A single domain can have multiple mail servers assigned to it. These multiple MX records have priorities assigned to each, which determines the order in which mail servers are chosen for e-mail processing.

The priority is determined in such a way that the lowest numeric value MX is treated as the primary mail server. If primary is down, the next priority is given to the next lowest value and so on.

In this sample MX record, the mail server ‘mx1.domain.com’ has priority 10 and is assigned as the primary. The mail server ‘mx2.domain.com’ with priority 20 would be chosen next, and so on.

MX records for a domain with backup MX

The benefit of having multiple mail servers associated with one domain is that the additional mail servers can act as Backup MX in case the primary mail server becomes non-functional.

Many reasons, such as connectivity problems, server downs, maintenance, security or software upgrades, hardware crashes, high server load, spamming, etc. can cause the primary mail server to be inaccessible.

When primary mail server goes down, mails will be routed to the backup MX, based on the priority set in DNS records. With the help of this backup MX, we can ensure that your customers receive their important mails in a timely manner.

At Bobcares, we configure and setup Backup MX servers for our customers as a part of the disaster management plan, to recover all customer mails in the case of any catastrophe.

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How to setup a Backup MX

Here, we’ll discuss the steps we follow to setup backup MX servers for our customers.

1. Setup an identical mail server

The first and foremost step in setting up a backup MX is to configure an identical mail server as the primary mail server, in another server. While setting up the backup MX, the following aspects should be checked:

i. Mailbox storage – Recipient mail accounts are configured in the backup server with enough storage space, so that users can check the mails without delay.

ii. Spam filters – The filters and forwarders present in the primary mail server are setup in the backup server also, to avoid messing up the custom email functionalities.

iii. Bandwidth usage – We choose the backup MX in a network that ensures business continuity, but at the same time reduces bandwidth costs.

iv. Secure transmission – A secure transmission channel is then setup between the primary and backup MX to transfer the emails securely.

v. ACLs – The blacklists and whitelists that are configured in the main server are copied over to the backup server, so that it knows which are the valid sender addresses to accept mails from.

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2. Configure low priority MX

After setting up the backup MX server, the DNS records of the domains should be updated with a new MX record, that points its secondary MX to the newly setup backup server.

The DNS entries can be added via the ‘DNS management’ interface for the users or from the backend, via custom scripts. The DNS changes may require a few hours to propagate worldwide.

Once the MX records start propagating, we perform a test of this setup by stopping the primary mail server and checking the email logs of the backup MX to confirm that the mails are re-routed correctly.

3. Sync the mails

Syncing of mails from the backup MX to the primary mail server is the final step. Once the primary mail server is up and functioning, the mailboxes can be synced from the backup MX to the primary.

But the backup MX should know when the primary server starts responding, inorder to sync the mails. We configure custom monitoring scripts in the servers to accomplish this.

The monitoring script in the backup MX use tools such as telnet to detect whether the primary is up or not, and then transfer the emails securely to it when it comes up.

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Conclusion

Backup MX are mainly setup to provide an uninterrupted mail service to the customers hosted in your servers. But they can also be used to load balance the mail servers.

Having two identical mail servers with equal priority MX records is useful for evenly distributing the incoming email load among two mail servers, in case one server load goes high.

To combat inbound spamming by diverting spam mails to secondary mail server and sending only valid emails to the primary mail server, we make use of the backup MX setup.

Bobcares helps online businesses of all sizes achieve world-class security and uptime, using tried and tested solutions. If you’d like to know how to make your servers more available, we’d be happy to talk to you.