The mailbox for /o=First Organization/ou=first administrative group/cn=Recipients/cn=USERNAME has exceeded the maximum mailbox size. This mailbox cannot send or receive messages. Incoming messages to this mailbox are returned to sender. The mailbox owner should be notified about the condition of the mailbox as soon as possible.
To change the default mailbox size limit, open the Exchange System Manager and navigate to "Servers -> SERVERNAME -> First Storage Group -> Mailbox Store" and right-click "Mailbox Store". Then, click on the "Limits" tab and change the limits if necessary.
You can also set per-user storage limits in "Active Directory Users and Computers" by viewing the user details and clicking "Storage Limits" in the "Exchange General" tab.