Hi, I'm currently planning the install of an Exchange 2007 CCR Cluster model to support 7500 users mailboxes. In our environment many users are light although we do have a reasonable number of large mailboxes. Probably only around 3000 people would ever be connected at the same time using methods such as OWA, Outlook online and Outlook cached. My initial thinking was to to have all 7500 mailboxes on a single server replicating as part of a CCR cluster to a second data centre. The more I read now though I am beginning to think that 7500 mailboxes on a single high spec server may be too many and that I should have 2 mailbox servers. Any advice appreciated. I understand that there is no firm guidance around maximum mailboxes per server etc. Stephen

Hi Stephen, Asingle CCR-based CMS can handle 7500 user mailboxes without a problem if sized properly (make sure you refer to http://technet.microsoft.com/en-us/library/bb738142.aspx). It's more about whether the business can afford having all users down at the same time should a disaster strike. What does your SLA say? I can assure you that with a maximum of 3000 concurrent sessions you will alsonot reach the store MAPI session or TCP/IP limitations (which you can read more about at: http://technet.microsoft.com/en-us/library/cc540453.aspx).