Mail delivery failed

He chequeado el sistema con 2 antivirus diferentes, le pase el Ad ware, el
Spybot e incluso el Registry Mechanic y aun asi cada cierto tiempo recibo en
mi cuenta de Hotmail un mensaje del tipo:

De:Mail Delivery System <Mailer-Daemon@easynet.es>
Asunto:Mail delivery failed: returning message to sender

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

olano21@marca.es
mailbox is full

This is a copy of the message, including all the headers.
The body of the message is 73489 characters long; only the first
65536 or so are included here.