Exchange SMTP high availability with Kemp load balancer

If you have more than one Exchange Server running in the organization, you want to load balance Exchange SMTP for high availability. An excellent load balancer that we do recommend is Kemp. This article will teach you how to load balance Exchange SMTP for high availability with Kemp load balancer.

Introduction

HTTPS and SMTP are both network layer protocols to transfer information between hosts. SMTP will transfer emails between mail servers, while HTTPS is used to transfer secure communication over a network.

We will load balance Exchange Server for the protocols:

• SMTP (Simple Mail Transfer Protocol) (this article)
• HTTPS (Hypertext Transfer Protocol Secure)

Important: Read the article Exchange high availability namespace design and planning before proceeding.

An architecture view of Exchange SMTP high availability is shown below in the diagram. It’s divided between External and Internal.

External

How it works if we send messages (emails) to an @exoip.com mailbox:

1. The public DNS/External DNS will retrieve the MX record for domain exoip.com
2. It will resolve to the cloud spam filter SpamBull.com MX records
3. The cloud spam filter SpamBull.com will scan the message for malware, spyware, viruses, and spam
4. SpamBull.com delivers only clean messages to the firewall’s public IP address
5. The firewall forwards the messages to the Kemp load balancer
6. Kemp load balancer distributes the messages load between the Exchange Servers

Note: Always use a spam filter to protect your Exchange Server organization from incoming and outgoing spam. The one we recommend is the SpamBull cloud spam filter.

Internal

If we send messages from internal it will directly go through the Exchange Servers and not through the load balancer. Only SMTP relay will send the messages through the Kemp load balancer.

How it works if we send messages (emails) from internal applications and printers:

1. The internal DNS will retrieve the A record for relay.exoip.com
2. It will resolve to the Kemp load balancer internal IP address
3. Messages are sent to the Kemp load balancer
4. Kemp load balancer distributes the messages load between the Exchange Servers

Before you start, read the articles:

• Install Kemp virtual load balancer on VMware
• Configure Kemp virtual load balancer
• Exchange HTTPS high availability with Kemp load balancer

Configure Exchange SMTP high availability

To configure Exchange SMTP high availability on the Kemp LoadMaster, follow the below steps.

1. Check Kemp LoadMaster Exchange templates

Sign in to Kemp LoadMaster load balancer to start.

In the menu, go to Virtual Services > Manage Templates.

We can see the Exchange templates we installed in the previous article.

2. Create new virtual service

Click on Add New in the menu.

Start first by selecting from the dropdown menu Exchange 2016 SMTP.

Specify the Virtual Address. In our example, it’s 192.168.1.54.

Click Add this Virtual Service.

Make sure that another device didn’t take that IP address. If you already have an old load balancer and want to replace it with Kemp LoadMaster, you can keep using the same IP address. The SMTP mail transfer will go through Kemp LoadMaster.

Click on Real Servers and then on Add New.

Add the Exchange Server IP address.

Click on Add This Real Server.

Add the second Exchange Server IP address.

Click on Add This Real Server.

If you have more than two Exchange Servers, add them with the same steps.

You can see which Exchange Servers you did add.

Click in the menu View/Modify Services to verify the virtual IP address with port 25 (SMTP). Both the Exchange Servers will show as Real Servers with the status Up.

3. Edit firewall VIP

In the firewall, change the VIP with protocol SMTP (25) to the Kemp virtual address. In our example, the IP address 192.168.1.54.

4. Check real time statistics

Click Statistics > Real Time Statistics in the menu.

Click Virtual services.

The Exchange Service SMTP shows the status Up, including the Exchange Servers.

Test and verify that it works

It’s always good to test the load balancer and check if it works as expected.

Disable the network card on one of the Exchange Server and check the statistics. The real time statistics will show the Exchange Server status Down.

Go to Microsoft Remote Connectivity Analyzer (MRCA).

Click in the menu on Exchange Server > Inbound SMTP Email.

Create a test user account with a mailbox and fill in the email address.

Note: You will receive messages from Microsoft Remote Connectivity Analyzer on that email after the test completes.

Enter the verification code and click on Perform Test.

Note: Don’t use an account with administrator rights.

The test is being performed. It will not take long.

The connectivity test is successful. The test did the following:

1. Attempt to retrieve DNS MX records for domain exoip.com
2. Retrieved MX records mx1.spambull.com, mx2.spambull.com, mx3.spambull.com, and mx4.spambull.com
3. Attempting to resolve the hostname mx1.spambull.com in DNS
4. IP address returned IPv4 132.117.53.188/IPv6 2001:978:2:2f::5:100
5. Testing TCP port 25 on host mx1.spambull.com to ensure it’s listening and open
6. Banner received from host mx54.spambull.com ESMTP

We can’t see the Exchange Server hostname because the SpamBull spam filter protects it.

Suppose you don’t have a spam filter for inbound mail, which we don’t recommend. The test will look as follows.

We did successfully configure Exchange SMTP high availability with Kemp load balancer. Don’t forget to enable the network card on the Exchange Server to bring it back up.

In the next article, we will configure Exchange outbound SMTP high available.

Keep reading: Add second domain to Exchange Server »

Conclusion

You learned how to configure Exchange SMTP high availability with Kemp load balancer. The templates are great that Kemp provides. Follow the steps to load balance Exchange inbound SMTP with Kemp LoadMaster. As of last, test the SMTP flow with Microsoft Remote Connectivity Analyzer (MRCA) or send an email and analyze the headers.

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