I just did an install of NodeBB 0.8.2 and it installed nodebb-plugin-emailer-mandrill 0.3.1 with it which is only compatible with NodeBB 0.9.^. In addition to the declared version problem I was also getting a socket.io/helpers related error on NodeBB startup until I removed the mandrill plugin.
I worked around this problem by removing mandrill and then installing it like this:
npm install email@example.com
I poked around to see how nodebb installs nodebb-plugin-emailer-mandrill and it appears that this plugin is installed in a special way? It is not mentioned in the package.json requirements for nodebb.
I would also like something like this for other reasons. We have a sub category where users post questions and we have certain people assigned to receive emails when new topics are posted - they are supposed to then go and reply to the question and it would be nice to be able inform them somehow that someone else (that was emailed) is already in the process of replying.