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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.

Information

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:

Important: Read the article Exchange high availability namespace design and planning before you go further.

An architecture view of Exchange SMTP high availability is shown below in the topology. 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

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:

Kemp LoadMaster Exchange templates

Log into Kemp LoadMaster load balancer to start.

Exchange SMTP high availability with Kemp load balancer 1

In the menu, go to Virtual Services and click on Manage Templates. We can see the templates that we did install in the previous article.

Exchange SMTP high availability with Kemp load balancer 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.

Exchange SMTP high availability with Kemp load balancer 3

Click on Real Servers and then on Add New.

Exchange SMTP high availability with Kemp load balancer 4

Add the Exchange Server IP address. Click on Add This Real Server.

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Add the second Exchange Server. Click 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.

Exchange SMTP high availability with Kemp load balancer 7

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.

Exchange SMTP high availability with Kemp load balancer 8

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.

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.

Exchange SMTP high availability with Kemp load balancer 9

Test and verify that it works

It’s always good to test the load balancer and 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.

Exchange SMTP high availability with Kemp load balancer 10

Go to Microsoft Remote Connectivity Analyzer (MRCA). Click in the menu on Exchange Server and follow with Inbound SMTP Email.

Exchange SMTP high availability with Kemp load balancer 11

Create a test user account with a mailbox and fill in the email address. Don’t use an account with administrator rights. Enter the verification code and click on Perform Test.

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

The connectivity test is successful. The test did:

  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.

Exchange SMTP high availability with Kemp load balancer 14

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

In this article, 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 header.

Did you enjoy this article? You may also like Mailbox still visible in Outlook after removing permission. Don’t forget to follow us and share this article.

ALI TAJRAN

ALI TAJRAN

ALI TAJRAN is a passionate IT Architect, IT Consultant, and Microsoft Certified Trainer. He started Information Technology at a very young age, and his goal is to teach and inspire others. Read more »

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