Thanks for your response. I sort of like the idea of restricting the content that can be sent by unapproved senders, and wonder if this could be adopted by email systems.

Suppose that the major email service providers and client developers agreed that from a certain date, all emails received from addresses that a user had not seen before would be truncated to 256 plaintext Unicode characters. The client UI would offer the option to whitelist that sender for future messages, which would also cause the existing message to be displayed in full.