The mailbox storage limit is the maximum allowable size of email that your account can receive. This can be set manually per user in Active Directory on the Exchange General Tab (Storage Limits), or you can create a global policy from within Exchange System Manager to control this limit.
Quota-based archiving archives items within your mailbox until the size of the mailbox is below the quota percent that was set. For example if you have a 50MB limit on your mailbox, and you set your quota percentage to 75%, then the archive task will archive items until 75% of your 50MB mailbox is free.
The only problem that we have found with Quota based archiving is that by default it archives 1,000 items per archiving run. So if you have a larger environment then you need to setup archiving to make several passes to keep up with the volume of email that should be archived based on your quota.
