Hi @RichardM50,
Great suggestion, this could be useful in reporting. The best place to make these suggestions would be the HubSpot Ideas section of the community. The product team reviews these suggestions based on popularity.
For an immediate solution, have you considered working with HubSpot templates? Create and send message templates It’s not exactly what you’re looking for but would allow you to report on certain types of mails. HubSpot has default reports for templates in the report library (Menu > Reports > Reports > Report library > select Templates in the left sidebar).
Hope this helps!