Ok. You have ton understand that neither Kmail nor Kontact are part of the Plasma project in any shape or form. The same way as Krita, Kdenlive, KDE Connect, GCompris, etc. are not part of Plasma. Plasma does not need any of them to be fully functional and, conversely, none of them need Plasma. Hell, most of them don’t even need Linux, being able to work on Windows, macOS, Android, etc.
Are KMail and Kontact dependant on Akonadi? Yes, but, again, Akonadi is not part of Plasma either. All are KDE, but they are all different projects, and none of them are Plasma.