Your external DNS is incorrect (the forward and reverse translations are mismatched), and that will cause some inbound and some outbound mail to be dropped by remote servers. That's not specific to the error reported here, but it'll cause problems.
As for this specific error, search around the forums. See this thread and this thread and probably a few others. See if ClamAV tipped over (launch Terminal.app and see if ps -U clamav finds ClamAV still running when this wedge occurs, and dig around in the logs for other errors), or whatever other content-processing is in use here.