Possible Solutions
One solution to this problem is to reset the global size limit to a large number and forget about it (in ESM, select Global Settings, Message Delivery Properties, Defaults). If you haven't set individual mailbox size limits and specific size limits on individual SMTP virtual servers, this option will work. However, it defeats the purpose of limiting people to specific message sizes.
Another option would be to isolate the public folder servers in their own, dedicated routing groups. You could then use a routing group connector to replicate the public folders between the dedicated routing groups and the underlying SMTP virtual servers, which can be configured with large size limits (e.g., 50MB). This approach applies the global settings to the mailbox servers and the SMTP bridgehead servers. However, it also introduces some complexities from a client perspective in terms of planning public folder access and failover (a discussion of which is outside the scope of this article).
You could also combine global settings and user-specific settings. For instance, you could set the global setting to a large value (e.g., 50MB), then apply per-user settings and smaller limits (e.g., 10MB to 12MB) for every mailbox in the organization. Quite a few large companies use this approach, which works well if you have a consistent provisioning process in place. To see how these per user settings affect the transfer service's categorizer component, you'd need to examine each recipient's settings. But implementing this solution and subsequently running the ExBPA tool will result in errors related to the message size limits, as Figure 3 shows.
You might find many other possible solutions. The important point is to understand how global settings and public folder replication work together. We chose to use a large global settings limit and implement a small size limit for the SMTP virtual servers on the individual mailbox servers. This solution let us continue to send large public folder replication messages through the infrastructure and disable large messages from being sent to or received from users or mailbox servers.
OAB Size Limits
If you don't use public folders to store large items, you might think you'll never encounter this problem. But in many environments, Offline Address Books (OABs) exceed the global message size limit settings, and OABs can be replicated to other systems in the Exchange organization. You can check an OAB's size limit by using ESM to drill down to Folders in an administrative group, right-clicking Public Folders, and selecting View System Folders, the OFFLINE ADDRESS BOOK folder, and the OAB Version 3a folder. Select the Content tab to view the items in the folder and check the associated attachment sizes. If an OAB is replicated, you'll need to specify—in the global settings or on individual virtual servers—the size of the information that will be replicated and whether message limits will affect its replication.
Understanding the Process
Unfortunately, Exchange doesn't let you apply the global message size limits to specific types of messages (e.g., user messages). Understanding how global size limits and public folder replication work with each other will result in fewer headaches later.