attachment is blocked in email

I send a email with an excel file attachment. I got a delivery error that says “The following recipient(s) could not be reached: klmx@streamyx.com on 09/03/2006 10:23 AM

You do not have permission to send to this recipient. For assistance, contact your system administrator.
The file Book1.xls has been blocked. File quarantined as: >

I send the same email with attachment to another person and he is able to receive it.

please advise,

thanks and regards,