net/mail: mail.ReadMessage in 1.20 cannot parse mbox headers [1.20 backport] #60874
Description
@ianlancetaylor requested issue #60332 to be considered for backport to the next 1.20 minor release.
@gopherbot Please open backport to 1.20.
This stopped working due to the changes in #53188, but those changes don't apply to net/mail.