Have the option Only allow collaborators to upload under "Enable email to folder" only modifiable by folder Owners/Co-owners.
Given that the "upload by email" setting is available to editors, making this change would restrict the possibility of an editor who didn't know what they were doing, from creating an email address that externals could use to upload to the folder.
It creates a bottleneck (which in this case is desirable) to create an 'open' email-to-folder address.
I suggest you make this ticked by default. Please also have some help text to explain the potential risks of unticking the box.