Something like
Notes4Email (which is for Outlook for Windows)?
I don't know of any for Mail on a Mac.
Also...these days, with such an "add-on" the note attachment only exists on your one device and isn't synced to a server. Additionally, a software update could "break" the add-on and cause one to lose all the notes...until if and when the add-on developer updates the software.
A more complex solution of Customer Relationship Management software that integrates with your business email could work and would probably allow you to make notes on emails as well as documents.