In exchange 2010 the quotas can be either set for the entire ExchangeDB, or per mailbox.
Atention ! If you set a quota on the DB, and then you set a quota on the mailbox, the mailbox setting will override the DB setting.

A. How to set a storage QUOTE for a Exchange Database, using EMC ( Exchange Management Console ):

1. Open the EMC, then expand the Organization Configuration / Mailbox, then go to Database Management tab. After that, right click on the database you want to change the quote to, and selectProperties.

2. In the properties window, go to Limits Tab, and change the Issue warning / Prohibit send / Prohibit send and receive according to your needs. Then click OK, and you`re done.

B. How to set a storage QUOTE for a specific user, using EMC (Exchange Management Console):

1. Open the EMC, then expand the Recipient Configuration / Mailbox, and then click on the desired mailbox you want to change the quote to, and then either right click / Properties or click on propertiesfrom the right menu.

2. In the Properties window go to Mailbox Settings tab, click on Storage Quotas and thenProperties:

3. In the storage quotas window deselect the Use mailbox database defaults, and then set your desired values for Issue Warning / Prohibit send at / Prohibit send and receive at. Then click OK / OK. And you`re all set up.

ENJOY.

And if you encounter any problems feel free to comment / contact me.