postfix - queue file write error

[humpy]

This has been driving me nuts, I have googled but nothing quiet seems to match this particular problem. I am running Centos5 with plesk, and the postfix config that came as part of plesk. I continually get a queue file write error as follows. Now I get this occassionally with other emails, but this one hotmail address sending seems to fail constantly. I have tweaked the setting as much as I dare, but cant seem to stop it..

Out: 220 vps.servermasters.net ESMTP Postfix
In: EHLO col0-omc2-s3.col0.hotmail.com
Out: 250-vps.servermasters.net
Out: 250-PIPELINING
Out: 250-SIZE 819200000
Out: 250-VRFY
Out: 250-ETRN
Out: 250-STARTTLS
Out: 250-AUTH PLAIN LOGIN DIGEST-MD5 CRAM-MD5
Out: 250-ENHANCEDSTATUSCODES
Out: 250-8BITMIME
Out: 250 DSN
In: MAIL FROM:<gumakuma@hotmail.com> SIZE=14523
Out: 250 2.1.0 Ok
In: RCPT TO:<tomo@wilsonarchitects.com.au>
Out: 250 2.1.5 Ok
In: DATA
Out: 354 End data with <CR><LF>.<CR><LF>
Out: 451 4.3.0 Error: queue file write error
In: QUIT
Out: 221 2.0.0 Bye

or alternate ending

Out: 354 End data with <CR><LF>.<CR><LF>
Out: 451 4.3.0 Error: queue file write error

Session aborted, reason: lost connection

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any suggestions gratefully received!.
thanks