Great question from my main homie Brian Hill:
Is there a backend DB reason for setting quotas at a certain size? I have found several links (like this one) discussing the need to set quotas due to the way the Outlook client handles large numbers of messages or OST files, but for someone who uses OWA, does any of this apply?
Short answer: no.
Somewhat longer answer: no.
The quota mechanism in Exchange is an outgrowth of those dark times when a large Exchange server might host a couple hundred users on an 8GB disk drive. Because storage was so expensive, Microsoft’s customers demanded a way to clamp down on mailbox size, so we got the trinity of quota limits: prohibit send, prohibit send and receive, and warn. These have been with us for a while and persist, essentially unchanged, in Exchange 2013, although it is now common to see quotas of 5GB or more on a single mailbox.
Outlook has never had a formal quota mechanism of its own, apart from the former limit of 2GB on PST files imposed by the 32-bit offsets used as pointers in the original PST file format. This limit was enforced in part by a dialog that would tell you that your PST file was full and in part by bugs in various versions of Outlook that would occasionally corrupt your PST file as it approached the 2GB size limit. Outlook 2007 and later pretty much extinguished those bugs, and the Unicode PST file format doesn’t have the 2GB limit any longer. Outlook 2010 and 2013 set a soft limit on Unicode PSTs of 50GB, but you can increase the limit if you need to.
Outlook’s performance is driven not by the size of the PST file itself (thought experiment: imagine a PST with a single 10GB item in it as opposed to one with 1 million 100KB messages) but by the number of items in any given folder. Microsoft has long recommended that you keep Outlook item counts to a maximum of around 5,000 items per folder (see KB 905803 for one example of this guidance). However, Outlook 2010 and 2013, when used with Exchange 2010 or 2013, can handle substantially more items without performance degradation: the Exchange 2010 documentation says 100,000 items per folder is acceptable, though there’s no published guidance for Exchange 2013. There’s still no hard limit, though. The reasons why the number of items (and the number of associated stored views) are well enumerated in this 2009 article covering Exchange 2007. Some of the mechanics described in that article have changed in later versions of Exchange but the basic truth remains: the more views you have, and/or the more items that are found or selected by those views, the longer it will take Exchange to process them.
If you’re wondering whether your users’ complaints of poor Outlook performance are related to high item counts, one way to find out is to use a script like this to look for folders with high item counts.
Circling back to the original question: there is a performance impact with high item count folders in OWA, but there’s no quota mechanism for dealing with it. If you have a user who reports persistently poor OWA performance on particular folders, high item counts are one possible culprit worth investigating. Of course, if OWA performance is poor across multiple folders that don’t have lots of items, or across multiple users, you might want to seek other causes.