[Dovecot] fcntl error

Error: imap(user): fcntl() failed with mbox file /var/mail/user: Resource temporarily unavailable

The user is using Outlook, I am not sure what the issue is, I am running Freebsd 6.0 and Dovecot 1.0.3

The user report that the gets about 60 of his inbox then I see this error in my error log. He is only running one instance of outlook

Doug