Resolution:
You would need to use the 64-bit version if you need to store several millions of messages. When messages are stored (even if they are stored on the disk), the server maintains a message "shell" in memory. Due to the address space of limitation on a 32-bit platform, the maximum number of pending messages is limited.

It is not possible for us to give a formula, we recommend instead that you test in your environment with real messages in a real case scenario and see what is more suitable for you.

Also, in order to have 64-bit EMS you have to have 64-bit OS.