X-Postfix; can't create user output file

I receive the following error when sending mail to one of my mailboxes:

Reporting-MTA: dns; ***********************
X-Postfix-Queue-ID: 4CCDBF9088C
X-Postfix-Sender: rfc822; ***********************
Arrival-Date: Thu, 1 May 2008 14:01:59 -0500 (CDT)

Final-Recipient: rfc822; **********************************************
Original-Recipient: rfc822; *************************
Action: failed
Status: 5.0.0
Diagnostic-Code: X-Postfix; can't create user output file

They can send mail okay, but not recieve mail.