Just dropping a post here for the Google search not to find in relation to issue that's posted a few times on forums here where editing any of the email templates gets a 403 Forbidden.

Can confirm (at least on Dreamhost) this is related to mod_security being enabled, and resolution on Dreamhost was to turn off 'Extra Web Security' which disables mod_security for that domain. Note that at time of posting Dreamhost specifically mentions that you can't disable mod_security with .htaccess. 

I have no affiliation w/ Dreamhost and would not necessarily promote them over any other host, this post just for information for anyone getting the 403 when updating email templates)