Social Engineering is a must

I think with all the technological advancements, on the hindsight we might have had significant benefits.
But,I strongly beleive that maintaining basic value and emphasizing amongst your employees is key for protecting vital data such as CEO's email.

Over and above that, the only way to implement encryption policies either using off-the-shelf products or create your encrption algorithm for protecting email.

I guess this non-standard disaster could be more damaging than a disaster due to other things.
This non-standard disaster will ruin the brand image once and for all.