I have a multisite set up here http://dmblogs.ncl.ac.uk
I've just set up dozens of student blogs and none of the registration emails have been received..
I've just tried this plugin http://wordpress.org/extend/plugins/wp-better-emails/ but still nothing..
I'd imagine the emails are stuck 'somewhere', but don't know enough about how wp processes the emails to know where to look.
Forgot password emails are working fine..