Whatever way you do it, throttling RAM for Exchange is a bad thing. Exchange is designed to use as much memory as it can get its "hands" on. You need to let it.
This means that you should never install Exchange on the same server as SQL Server, for instance, since this will result in serious memory contention issues, and cause significantly degraded performance. Similarly, co-locating Exchange on a domain controller or global catalog server is not recommended.
Speaking of RAM, don't forget to pay attention to well-known tuning recommendations
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