Another thing to consider: Find a v6 tunnel broker, and get a dynamic v6 tunnel for home semi-locally, and then just use v6 from your backup MX server to your primary server. I have tunnels from sixxs.net, and my user account and one tunnel go back five years. The only problem has been that occasionally the tunnel doesn't work usually due to some issue at the POP, but outages that last long enough for me to really look into them are quite rare. (sixxs.net does expect people to know what they are doing, which is fair enough for free service) sixxs offers aiccu-based tunnels (pkgsrc/net/aiccu), a dynamic protocol with an open-source client (but not server as far as I know). The client automatically establishes a tunnel, and it works over NAT (IPv6 in UDP in IPv4). The tunnel is not encrypted, but then your native v6 wouldn't be. I know several people/companies using a he.net tunnel, which have also been reliable. There are other tunnel brokers. One caution is that there are sometimes v6 peering disputes. I have heard of two (non-home, largish) places obtaining he.net tunnels because of this (ending up multihomed, so that all of the v6net is reachable via one or the other). Note that with sixxs.net, you get a prefix from the ISP that donates POP space to them, which in the US may be occaid, which I recall being hard to get to from cogent. This is not sixxs's fault of course. I have an occaid-based tunnel, and it's been working pretty well.
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