Exchange 2007 receiving emails with no recipient

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http://serverfault.com – I have a exchange 2007 mail server running on SBS 2008. I have an issue with some emails from some senders being received by the exchange server but not being forwarded to the recipients email inbox. This seems to be happening more on emails with attachments but no bounce backs or delivery rejection notices are being received. The server is running AVG 2012 for email scanning. Screenshot of message tracking below http://dl.dropbox.com/u/80787655/SYR_Exch_issue.png Any ideas on where to go to resolve / troubleshoot this would be great (HowTos)