Occasionally I have received the following error when trying to send emails to other organisation from Exchange 2007:

Diagnostic information for administrators:
Generating server: exchange-server.internal.domain.com
enduser@externaldomain.com
external.mail.server.domain.com #500 Firewall Error ##

It turns out that this is actually due to the Cisco router inspecting SMTP traffic. Entering the command below into IOS fixed the issue:

no ip inspect name SDM_LOW esmtp