This is a privacy-by-design decision:
Even though metadata is encrypted with XSalsa20-Poly1305 (very secure), the email is first hashed with SHA-256 before being encrypted.
This double protection ensures that even if encryption is broken in the future (e.g. via quantum computing), the original email is still not directly exposed.
It also allows the user to verify ownership by hashing their own email and comparing it with the decrypted value, without revealing it in cleartext.