Incoming SMTP traffic in your IPv6 tunnel (either 6to4 or 6in4) will not likely be blocked by the ISP since it's in a tunnel. Unless they're doing some sort of deep inspection of the traffic (e.g. looking into the tunnel payload at the IPv6/TCP packets) it won't block it.
Of course! Was I stupid...! It's evident once told out, thanks.
I bet the ISP wouldn't even think of inspecting proto 41 IP packets, let alone know how-to at the moment.
Not that a IPv6-only MX would be very useful either, would it ? Will regular sending SMTPs on the cloud interact with them successfully ?
For IPv4, you'd either have to tell them not to do it for you (it's an option with my ISP's web management interface), or you'd also have to tunnel IPv4 traffic from somewhere else into your MX.
My for-the-Mass type ISP won't do case per case exceptions. As for your second suggestion, would you kindly precise what's in your mind ?