Outgoing SMTP smarthost
Implement a smarthost to complement the NAT64 gateway, although ideally also allowing relaying of messages to IPv6 destinations.
Restricting sender domain to one owned by customer of sending machine
Antispam filter operating on similar principals to those used by major email service providers
Means for customers to register their domains for use for outgoing messages from their VM (and/or generation of list automatically from DNS)
Optionally, multiple servers with stricter/looser spam-like message filtering, allowing customer to choose their own balance between freedom of expression and reputation