This categorization seems to be based on one dimensional thinking (not in a bad way).
If you bring in more dimensions you end up with something like a taxonomy where an email contains a message and maybe also a todo or something else and may have more attributes around those identities. Then you can filter by those.
I do not use that personally but I see there is no reason to define an email as a todo item.
Also, as an email is just a storage for data, you could (mis)use it as a storage backend for a file system (see fuse) or backup solution.
If you bring in more dimensions you end up with something like a taxonomy where an email contains a message and maybe also a todo or something else and may have more attributes around those identities. Then you can filter by those. I do not use that personally but I see there is no reason to define an email as a todo item.
Also, as an email is just a storage for data, you could (mis)use it as a storage backend for a file system (see fuse) or backup solution.
So an email can be everything you want.