We've been having problems with sending emails to yahoo hosted domains, and whilst I've requested more info from Yahoo accordingly, I'm concerned there's an issue our end... can anyone deduce anything from the following entry in our zimbra.log?

Apr  3 19:25:31 server postfix/qmgr[29261]: B2E6EDC8011: from=<suyym@yahoo.com>, size=5965, nrcpt=1 (queue active)
Apr  3 19:25:31 server amavis[21489]: (21489-05) ESMTP::10024 /opt/zimbra/data/amavisd/tmp/amavis-20130403T190642-21489: <suyym@yahoo.com> -> <lucy@noahsarkdaynurseries.co.uk> SIZE=5965 BODY=8BITMIME Received: from server.mydomain.co.uk ([]) by localhost (server.mydomain.co.uk []) (amavisd-new, port 10024) with ESMTP for <lucy@mydomain.co.uk>; Wed,  3 Apr 2013 19:25:31 +0100 (BST)
Apr  3 19:25:31 server amavis[21489]: (21489-05) Checking: ca2hwLR9XP3B [x.x.x.x] <suyym@yahoo.com> -> <lucy@mydomain.co.uk>
Apr  3 19:25:35 server amavis[21489]: (21489-05) Blocked SPAM, [x.x.x.x] [x.x.x.x] <suyym@yahoo.com> -> <lucy@mydomain.co.uk>, Message-ID: <20130403043959.6332.qmail@bhocxalgii>, mail_id: ca2hwLR9XP3B, Hits: 27.798, size: 5962, 3979 ms
I've removed our domain details (just in case) and replaced it with mydomain.co.uk, and our public IP is replaced with x.x.x.x !

Can anyone give any advise whether there's a computer on our network with a virus etc, or something else ?!