[jeffbee]

All address literals other than IPv4 must be prefixed by a tag, such as "IPv6:". The form that I gave is a syntactically valid v6v4 address literal.

https://datatracker.ietf.org/doc/html/rfc5321#section-4.1.3

[thwarted]

Interesting, TIL. That makes the conversion to @2001 even stranger.