WordPress email alerts

Hi – any advise from members on this issue appreciated :

A user of one of our WordPress websites is complaining that they subscribed to receive update emails when posts are published on our site. However, they say they are receiving up to 10 emails per day from our site relating to old post published years ago.

Is there a plugin that will “catch” or report on email that is sent by the site so we can audit what has gone out to which users each day, and try and diagnose the issue? Or is this more of a server side thing.

Best

Marcus