Inconsistent Email When Someone Subscribes

I’ve only received one email out of five attempts when a member signs up. Even then, the email I received just said he was joining – it gave me no option to ‘approve’ him and he is still unable to ‘sign in’. I’ve triple checked my settings and email address – still four attempts failed. I even tried a new theme (stupid I know but wth) with no change. What’s with the inconsistency? What am I missing? This seems very wrong.