Microsoft Exchange Server 2016 Scalability

Microsoft Exchange Server 2016 is based on two server roles: Mailbox and Edge Transport. Mailbox servers host the mailbox databases that store emails for the users.  They also process and route email within the organization and proxy client connections to the mailbox server that currently hosts the active database for the user. Edge Transport servers are located in the network DMZ where they route inbound and outbound email with external networks.

Let’s discuss the scalability of each server role in turn:


Mailbox Server

A Mailbox Server can be standalone, or it can be a member of a Database Availability Group (DAG). Standalone servers host active mailbox databases but do not provide any High Availability (HA) protection against a server failure. Exchange DAG’s provide high availability and site resilience for the Exchange environment.  Each DAG can contain up to 16 Mailbox Servers, and each Mailbox Server in a DAG can host up to 100 databases in any combination of active or passive database copies. However, each server cannot have more than one copy of any particular database, and a maximum of 16 copies of each database is permitted in a DAG. Each DAG starts as a single node with additional nodes being added or removed as needs change over time. Exchange Server 2016 supports scaling the server hardware to meet additional requirements when hosting many databases in a DAG. Up to 24 CPU cores and 96 GB memory are possible in each Server if required. Additional hardware requirements should be spread out over other Mailbox Server instances in order to provide a solution that scales to your organization’s needs. With the required number of cores and memory in each server.

Mailbox Servers also service all client requests in the organization. Since all client connections use a single name space a load balancer can be used to distribute client requests to the Mailbox Servers. If an organization has a large number of client connections, then dedicated Mailbox servers should be used for client connections without hosting any mailbox databases.

Similarly, if an organization has a high volume of inbound and outbound email traffic then dedicated mailbox servers that do not host any active mailbox databases or client connections could be used for mail flow without impacting the mailbox database or client access performance. Exchange Server 2016 load balances all the transport mail flow across servers within the organization.


Edge Transport Server

Edge Transport servers are Internet facing and are placed in the network DMZ to route all inbound and outbound email traffic. They are not Active Directory domain joined servers. Edge Transport servers sync with the transport service on Mailbox Servers to send and receive email. They also perform one-way sync of recipients and configuration information from Active Directory via Mailbox Servers. Edge Transport servers protect the Mailbox Servers from viruses, SPAM, and also do content filtering.  High Availability is provided for Edge Transport servers by adding more than one of them with the same DNS MX report priority for them all. Additional Servers could also be introduced to handle increased load. Additional Edge Transport servers can be added quickly by cloning an existing one using supplied cloning scripts in Exchange Server 2016. 



Microsoft Exchange Server 2016 is a very scalable messaging platform at the hardware and software level. It’s important to ensure that any hardware changes made on an Exchange DAG node are applied to all the nodes in the DAG. If a new DAG is introduced into the organization then the Client Access service and Transport Service configuration should be replicated from the old DAG nodes to the new node.

Doing this will give you an Exchange Platform that is scalable as you users grow, is resilient to hardware errors, and that will allow for planned maintenance without service interruption.