Exchange 2003 Server: One User unable to receive but can send emails.

Hello,
Can someone help me here.
I have one user who is unable to receive emails in her mailbox but can send, be it through web or mail client (Ms Outlook). When I send her an email, I don’t receive a delivery failure notification message. I suppose that the message got sent. She can not receive mails sent from neither external nor internal sources, but can send to either.
We use Exchange 2003 Server, I have checked her mailbox size, it is still below the sending and receiving limits.
Thanks.