Keep Exchange In-House and in the Cloud With Coexistence

Organizations interested in some of the benefits of cloud computing have the option to run Exchange Servers in-house, but host mailboxes at Microsoft’s data center. This is known as “co-existence,” and it has its pros and cons, both from an IT manager’s and end user’s perspective. In this article, we’ll consider the the implications of co-existence for both groups.

The advantages of Exchange coexistence

The advantages of Exchange coexistence can be enumerated by looking at the benefits of each half a coexistence deployment.

An on-premises Exchange server managed by in-house IT enjoys these advantages:

  • Local emails sent/received more quickly
  • Lower WAN bandwidth requirements for HQ users
  • Data leakage minimized
  • Predictable maintenance

An off-premises Exchange server managed by Microsoft enjoys these advantages:

  • Cheaper hardware maintenance
  • Lower WAN bandwidth requirement (if lots of mobile users)
  • Highly secured
  • Predictable cost
  • Datacenter/Hardware redundancy

Let’s consider these advantages in more detail:

On-premise Exchange: Full control and faster performance

  1. Frequently used email recipients: Most emails that users send out are addressed to their colleagues, which usually reside in the same mail server or local network. By routing these emails within the same intranet insures faster delivery.
  2. Less bandwidth: Depending on the email usage pattern of the users, having on-premise means less WAN bandwidth needed.
  3. Confidential data stays within the network: Having everything on-premise ensures that confidential emails between colleagues stay within the company’s network.
  4. Maintenance period is transparent
  5. IT and management needs to know what’s happening with the IT infrastructure. Usually having things outsourced means that we only can rely on the reports by the vendor, and the downtime is beyond the customer’s control.

Off-premises Exchange: Services managed by the experts

  1. Lower cost: Paying a small fee to get features like hardware redundancy, datacenter redundancy and managed support is a no brainer. What more do you want when you do not need to setup and maintain these infrastructure.
  2. Very secure: Microsoft’s datacenter is equipped with 7 layers of defense-in-depth, from the physical layer to the network layer to the application layer, ensuring that each and every layer is protected with the necessary security.
  3. Predictable budgeting: You know how much email and other IT services will cost you every year. There will not be yearly hardware upgrades or maintenance for the mission critical applications. Management will be able to foresee and allocate the appropriate budget for mission critical applications.
  4. Redundancy for natural disaster: Microsoft has the firepower to ensure that customers’ data are replicated throughout a few datacenters around the world. This is to ensure redundancy of data if anything happens to one location. This infrastructure is not something that any companies can afford to build.

The disadvantages of Exchange Coexistence

Deploying either on-premise or using hosted emails have its benefits, but when you combine them together, it also means increased cost. Here are some disadvantages to consider:

  1. Increased cost: Organizations are no longer paying only for costs incurred for their on-premise Exchange; they are also paying for hosted infrastructure. Organizations will be paying twice the money to use email facilities, and this usually will trigger CFO’s attention.
  2. Increased management: There’s only one way to manage Microsoft Online Services: Log support tickets. If you have an issue, you log a support ticket for Microsoft Online Support to assist. Now that you have both Microsoft Online Services and on-premise, it means IT will have double work as they will need to manage both on-premise and Microsoft Online Services support tickets.
  3. Increased complexity: Co-existence involves integration of two different systems. If the bridge server (the server which copies the email from the local server to the remote server) goes down, emails will be affected. The ability to manage this kind of infrastructure requires IT that has good foundations of how email infrastructure works.

When does coexistence work?

There are several scenarios we can consider when deciding whether we might want to go fully to the cloud, fully on-premises, or try the blended approach coexistence offers.

Scenario 1: Remote work force, local IT management and staff

My sales teams are always out on the field, and they normally do not work from office. On the other hand, I have admin and management staff working in the office.

Co-existence is right for you. Your sales teams connect to Microsoft Exchange Online directly, lowering the WAN requirements for your office network. In the meantime, your office users will connect back to the on-premise Exchange Server for fast email communications within internal employees.

Scenario 2: HQ with small branch offices

My organization has a HQ with 50 users, and a few branch offices. Each of these branch offices only house less than 10 users.

Cost wise, it’s best to do co-existence. Reason being is, it does not make any sense to have the branch offices connecting back to the HQ for emailing, as this will increase the WAN usage for HQ, hence internet upgrade is required. Purchasing extra Exchange Server for each branch office will push up the cost tremendously, which is totally not an option.

Scenario 3: Larger HQ, no remote users or branch offices

My organization has more than 50 users, all under one roof.

No, do not do co-existence. Either host your mail server on-premise, or deploy BPOS fully.


Co-existence has pros and cons, but in the end of the day, management’s decision boils down to the dollar sign. So what I would suggest is to convert the above scenarios (or your scenario) into dollar sign and weigh whether do host it on-premise, off-premise or co-existence.

Do you have any other scenarios that you are wondering whether you should do co-existence? Or any other scenarios that you wish to share? Feel free to comment back!

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