After a few days head-scratching, I’ve finally been able to isolate and replicate the issue I’m having on an Exchange 2003 server (part of SBS 2003 stnd).
Any email with any attachment (zip, pdf, jpg, etc) will not get sent to any domain. If a plain text email or a standard html email gets sent to the same domain, it will be sent, and a delivery report generated for user etc.
At first i thought it was a DNS issue, but have ruled this out, as we've run testexchangeconnectivity, exchBPA plus manually reset DNS forward zones using Open DNS rather than the ISP's dns.
The exchange is set to route email via DNS, not a smarthost.
I have ruled out AV interference, we've clean installed AVG 2011, and the exchange scanning module has NOT been installed.
We also run an auto signature software, exclaimer, but this has been ruled out, as i disabled it for an hour during the day, and still got the same results; emails with no attachments sent ok, emails with attachments stuck in retry.
I have followed an article which suggestions a corrupt exch DB. Having run eseutil and verified clean shutdowns, no other errors, i have also run an isinteg which shows no errors, and an offline defrag. All mounted again, but still same stuck emails with attachments.
I am aware of a registry entry which affects the av scanning of exchange, but haven’t been able to find a reliable source of the key to edit.
The settings in Exchange do allow attachments, and the size limit is set as 'any' size in Global \ Message Delivery and is not restricted in the smtp connector. Limit message size is not set in default virtual smtp, and the users allowed email size is using system default.
Any help would be welcome now, as i've gone through most of the obvious (i think) problems. There were a few updates applied on 31st Oct, one of which was for Exchange (KB907747) this was for IMF.