The shared mailboxes will be subject to quotas. Each shared mailbox will be initially allocated 5 megabytes. Note that this will not impinge on your personal quota. This depends on the mail client you are using. Neither Simeon nor Pine allows you to ask about your quota usage, but both will inform you when you approach or exceed your allocation.
Messages continue to be delivered so long as any amount of quota is available.
When a quota is exhausted, however, messages will remain queued for delivery until such a time as either space becomes available or the messages time out. If this happens, they will be returned to the associated mailing list owner . This would allow IMAP mail folders to be referenced using URLs.