You can use the following table to estimate the memory requirements of a specific Mailbox server on a given node in a DAG. These recommendations are based on single node failures for small DAG’s (2 - 3) and double node failures for large DAG’s (4+). When you are configuring memory based on these two tables, always make sure to configure for max memory. For example, if mailbox size is +10GB, then you would size with 10MB of memory.

Mailbox sizing and memory recommendations

Recent x64-based servers have the ability to scale their memory configuration to 64 GB and beyond. There are several reasons why we do not recommend maximum memory configurations beyond 64 GB for Mailbox servers, including cost, the impact of non-transactional disk I/O, and cold state operations:

• Cost   Based on current memory prices, specifically the current average price of 4-GB dual in-line memory modules (DIMMs), it is cost prohibitive to install more than 64 GB on a single Mailbox server. Generally, the cost of physical RAM is linear up to 64 GB. Beyond 64 GB, the cost trend is exponential, and for many configurations, it is less expensive to add disk drives instead of memory.
  
• Non-transactional disk I/O   The Mailbox server utilizes additional physical RAM by caching more data, which reduces the database disk I/O footprint for transactional disk I/O (I/O that is generated by client activity). There are several sources of non-transactional disk I/O on the Mailbox server. These include online maintenance (for example, online database defragmentation); offline maintenance (for example, offline database defragmentation or database repair operations); backup, restore, or recovery operations; and mailbox management operations. All of these operations require disk I/O to properly maintain the server. Although Exchange 2010 has reduced transactional I/O significantly, adequate storage performance is still required for proper maintenance of the Mailbox server. For this reason, there is a point of diminishing returns when adding memory to the server. In general, the purpose of adding memory to the Mailbox server is to reduce the disk I/O requirements, which in turn reduces the overall cost of the storage solution. Due to non-transactional I/O requirements, the storage requirements of the server may not be significantly reduced by adding more than 64 GB of memory.
  
• Cold state operation   Cold state is defined as the state of the Mailbox server immediately following a server restart or a restart of the Microsoft Exchange Information Store service. The database cache, which is used to cache read/write operations, is small in size (cold) during this period, so it has a significantly diminished ability to reduce read I/O operations. As the Mailbox server processes messages, the database cache size grows, increasing the effectiveness of the cache and subsequently reducing disk I/O on the server. The more physical memory in the server, the longer it takes the database cache to reach its optimal size. If the storage solution is designed and sized for a server with a large amount of physical RAM (greater than 64 GB), and the disk I/O profile of the users assumes an optimal database cache state (for example, a large, warm cache), the client experience may be compromised due to insufficient disk performance during the cold state periods. Similar to the issue of non-transactional I/O, the storage requirements may be the same for a server with 64 GB of memory as a server with more than 64 GB of RAM. On a properly configured Mailbox server, it should take about 15 minutes to reach the optimal cache state after a cold operation has occurred.
  

Although the Mailbox server role will utilize memory greater than 64 GB, for the reasons outlined earlier, 64 GB is the maximum recommended memory configuration and is considered the point of diminishing returns in terms of both cost and performance.