I just wanted to make a note of this because it is easy to do via the Exchange Management Shell but can become problematic through the EMC, especially with a large number of mailbox databases. Essentially what we are looking to do here is change the default warning and prohibit limits that Exchange uses for user mailboxes.
The following command will change the warning size to 7GB and prohibit users the ability to send messages at 8GB.
Get-MailboxDatabase | Set-MailboxDatabase -issuewarningquota 7gb -prohibitsendquota 8gb
And we can double check our handy work through the EMC to make sure that we have these properties adjusted properly.
Everything looks good. I should note that there are a number of other really handy things that can be changed via the set-mailboxdatabse cmdlet and the ability to pipe it through get-mailboxdatabase is fairly straight forward, making global changes to your Exchange environment such as this one much quicker and easier to do. There is some good reading here:
http://technet.microsoft.com/en-us/library/bb123971.aspx
http://technet.microsoft.com/en-us/library/dd297937.aspx
