"It would be silly if anyone could win the whole war at any time just by owling him a hand grenade," Harry thinks in Ch. 37. A Fate Zero style conflict between a sane and a non-sane wizard ends very quickly, and if two sane wizards ever fight each other...
Rowling said somewhere in an interview that there were ways of making yourself impossible for an owl to find; whether this is a blanket affect or could be lifted for select targets is unclear. Otherwise, the Aurors could have just owled the Death Eaters something and then followed the owl to their hiding spaces.
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: