We have a Release 8.0.2.GA.5569.UBUNTU10.64 UBUNTU10_64 FOSS edition production instance.

This morning it suddenly became unable to process mail. It produces the "file write error" as soon as the recipent address is entered in Telnet. Using the web interface to send mail results in a "RCPT failed: Invalid recipient admin@example.com: 451 4.3.0 Error: queue file write error".

(example.com used as domain here for privacy reasons)
220 mail.example.com ESMTP Postfix
helo mail.example.com
250 mail.example.com
mail from:admin@example.com
250 2.1.0 Ok
rcpt to:simon@example.com
451 4.3.0 Error: queue file write error
I have checked that the disks aren't full, the permissions on the cache directories look fine. I ran zmfixperms just in case. The file systems seem to be mounted rw. I can create test files on each mounted partition. There are no errors in the zimbra.log, only errors in the mailbox.log as the Mailbox tries to call Postfix to send.

I can't think what would have suddenly changed on the server in the last two hours to get it into this state. Searching Google draws a blank. I have never seen telnet failing straight after the rcpt address is entered. That's weird.

Would appreciate any ideas or pointers where to look.