I wanted to test quota limits on a test user and set up the follwing: [PS] C:\>Set-Mailbox jcochrane -UseDatabaseQuotaDefaults $false -IssueWarningQuota 15KB -ProhibitSendQuota 20KB -ProhibitSendReceiveQuota 25KBI thought: "it won't take many messages for this person to reach their limit".Yet, I could exceed all the quotas (in particular the last two) without the user not being able to send mail:(Note: I'ved edited the output to faciliate reading) C:\>Get-Mailbox jcochrane | select UseDatabaseQuotaDefaults,IssueWarningQuota,ProhibitSendQuota,ProhibitSendReceiveQuota | fl UseDatabaseQuotaDefaults : False IssueWarningQuota : 15KB ProhibitSendQuota : 20KB ProhibitSendReceiveQuota : 25KB [PS] C:\>Get-MailboxStatistics jcochrane | fl AssociatedItemCount : 6DisplayName : Joe Cochrane ItemCount : 15 LastLoggedOnUserAccount : ACME\jcochrane StorageLimitStatus : BelowLimit TotalDeletedItemSize : 0B TotalItemSize : 27,932 B StorageGroupName : First Storage Group DatabaseName : Mailbox Database (Note: yes, I added the comma and spacing in the TotalItemSize parameter)It is only the day after that the user, having logged on to OWA, received the warning and could not, indeed, send (or recieve) messages.So it looks like Exchange verifies the quota limits every 24 hours (?) and then takes action, warning or blocking the sending of messages as the case may be?

http://technet.microsoft.com/en-us/library/bb684892.aspx Mailbox Size Limits Are Not Enforced in a Reasonable Period of TimeRestarting the IS will enforce the change immediately.