Mailbox Quota Limits

When I enable mailbox quota limits my users loose messages whether I configure the storage limits to “prohibit send and receive” or “issue a warning”.
The Outlook 2003 client has POP and SMTP configured.
Exchange 2003 Enterprise server does not have the POP3 service enabled and it is not configured to foward any unresolved messages.

I’ve checked the queue and still can’t find the message.

Anyone with ideas?