Time for another crm goodie...
We've been working on a little project that needed blocking of certain outbound emails, with the Dynamics CRM plugin architecture this made it very very simple. For whatever reason, if you want to block outbound emails from Dynamics CRM programmatically, hook into the Send message on the email entity at the Pre stage synchronously then set the IssueSend property to false.
A bit of code
A little gotcha, the system marks the email as Sent even though we've blocked it, so be careful with that one. Hopefully this is useful to someone else. We've used this technique to write a neat little addon for Dynamics CRM, keep an eye out for it!