As mentioned on bug 52915, we don't have any stats on how many emails we're sending from MediaWiki or otherwise, we have no idea of the impact our choices have though it would of course be quite important to assess it e.g. to have at least a slight idea of how likely we are to be considered spammers. As written elsewhere:
Note that we used to have some very aggregate statistics, made by Mark I believe, on the number of mails sent by our mail relays. They died some years ago though, we only have things like
Restoring mail relay statistics should be easier than proper eventlogging and could give some general clue, but it's just a guess I'm making.
Ori suggests we could just use whatever command to get the numbers and then send them to ganglia as we do for the job queue stats. mutante points to http://linux.die.net/man/8/eximstats which seems to be part of the standard packages http://packages.ubuntu.com/precise-updates/amd64/exim4-base/filelist