A subject access request is indeed normally made when somebody wants to find out what personal data an organisation holds about them. The standard reply should include, confirmation it is processing their personal data, a copy of that personal data and details of how that data is collected, used and disposed of.
I made one once to a former employer and I was sent copies of documents that had sickness absence, my qualifications, notifications about birth of children, change of address, entitlements & contributions in the company pension scheme etc.
It would appear that the person concerned is actually seeking information which falls under the freedom of information for correspondence about a specific topic in which probably their name only is simply cited. Their name could potentially appear dozens of times but in the context of an SAR you could reply that you hold their Title, surname and Christian name which have been cited in X Email messages. Probably OK to provide a redacted copy of just one of them since just one would be all that would have been needed to get the details since the messages probably don't refer to their date of birth, ethnicity or images etc. My advice would be to reply to the requestor and tell them that the information they seek does not appear to fall within the scope of a subject access request, and that they consider asking for the information in the form of a Freedom of Information Act request.