Email encryption would be a nice feature and i guess it could easily added by letting the Admin/user specify a private key for the zabbix-server and a public key for the send_to user. => Zabbix could then encrypt every mail with the public key of the recipient, which could make sense in some environments, since mails from zabbix may contain sensitive information.
But first: Is there a genereal interest in this idea? What do the devs think?