That is by design; Exchange will populate it’s store cache as messages are processes.
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 32 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 32 GB of memory as a server with more than 32 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.
Make sure you have properly designed your memory specs
2GB + 2-5mb per user
Deploying a Simple Exchange Server 2007 Organization
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Our store.exe process has been consuming more and more memory. We have 10 GB installed, and the store is using almost 7. This pushes the total memory used for the server to over 10GB, and is slowing it down. When I reboot, store starts off much smaller, but over several weeks, it inches back up again.