I recently made the assumption (not very accurately as it turns out) that deploying large mailboxes of a GB and above in online mode is not generally realistic. I think it’s a fairly common misconception that having a local cache is always going to improve Outlook performance. Well actually that’s not entirely true. For mailboxes under 1GB then there are likely to be improvements in search, changing views – that sort of thing. ..but beyond 1GB the performance of an .ost is thought to degrade quite markedly up to about the 2GB mark where stability and performance mean the size of the .ost reaches its recommended limit.
This should not deter you from deploying cached mode given the right circumstances by the way. Cached mode is particularly useful for mobile users like myself who regularly switch between online and offline mode; and of course it’s essential where network connectivity between client and server is either intermittent or demonstrates significant latency. This whitepaper; Plan a Cached Exchange Mode deployment in Outlook 2007, is a useful read if you are considering deploying Outlook in cached mode.
A key bit of information that has recently matured slightly is the recommendations concerning the number of items in the critical Outlook folders. I think it’s pretty well understood now that it’s is item counts rather than item size or mailbox size that has the greater impact on Outlook performance. For Exchange 2007 it’s essentially the same engine at the back end and but the guidelines are now higher. About 20,000 items rather than 5,000 items is the more up to date guideline. Of the Outlook pilers out there there are a lot with inboxes containing 15-20,000 items and they can work in online mode without much difficulty at all…
Of course having said that this all comes back to decent design… With E2K7 it is perfectly possible to deploy mailboxes of 1-2GB (and a long way beyond) providing you size the server and the storage appropriately. You may have to compromise on the number of mailboxes you deploy per server but that shouldn’t necessarily deter you. Offering very large mailboxes on more cost effective storage, for example, might mean you recoup investment elsewhere in the platform. You may well satisfy your users requirements by deploying 4GB mailboxes but save the pot of money required to support your terabytes of .pst’s and your email archive solution..?
So are big online mailboxes realistic? ..absolutely.