Roland Turner

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MX6, An Immodest Proposal for Easing the Acceptance of Email over IPv6

Most current efforts to start accepting email over IPv6 at scale appear to be stumbling on two irreducible problems:

If these problems are the relevant ones and are truly irreducible then what is required to allow more early adopters to start is a way for a sender to affirmatively indicate its ability to interpret a "fallback to IPv4" response code, even before an AAAA record is returned when looking up the MX; if it doesn't indicate its ability to do so then only an A record should be provided, meaning that delivery will only be attempted over IPv4. Proposed here is a means of doing so by specifying a new Service name for an SRV record ("MX6") and the sequence of actions around a new response code.

A receiver NOT implementing MX6 might publish the following:

only4.example           MX   0 mx.only4.example
mx.only4.example        A

while a receiver implementing MX6 might publish the following:

with6.example           MX   0 mx.with6.example
mx.with6.example        A

_mx6._tcp.with6.example SRV  0 0 25 mx6.with6.example
mx6.with6.example       A
mx6.with6.example       AAAA 0:0:0:0:0:ffff:a00:2 

This would support the following scenarios:

An MX6-unaware sender delivering to an MX6-aware receiver

An MX6-aware sender delivering to an MX6-unaware receiver

Both MX6-aware, receiver is willing to accept the message over IPv6

Both MX6-aware, receiver is unwilling to accept the message over IPv6

Further thoughts:

Comments welcome.