Developers original post is here. Copied below for convenience.
Problem is that NextGEN is loading the following file in its constructor:
That files loads the whole library of WordPress pluggable functions which have not been loaded by the time it is included. Unfortunately wp_mail is one of those functions that gets loaded.
So plugins fail (wpMandrill or NextGEN or whatever) when they try to override any pluggable function after NextGEN has been declared/loaded. wpMandrill, at least, fails gratefully! ;)
They should try to makes NextGEN load after all other plugins have been loaded, or implement a lazy/delayed loading in their plugin.