RFC 8601: update reading of Authentication-Results to new RFC

[lieser]

Current logic for Authentication-Results header is based on RFC 7601.

There exist a newer RFC 8601. No analysis yet done what changed, and how it affects the add-on.

[lieser]

Relevant changes between RFC 7601 and RFC 8601:

• Added IANA registration for DKIM "a" and "s" properties. (https://www.iana.org/assignments/email-auth/email-auth.xhtml)
  • Section 2.7.1. / 6.3.1. / 6.3.2.
  • Could potentially apply the error/waring setting about usage of rsa-sha1
• Added support for Internationalized Email (see also #163)
  • Section 1.5.2.
  • Formal Definition changes
    • "local-part" as modified by RFC6531
      • used in pvalue part of propspec
    • "value" is as defined [...] with "quoted-string" updated as specified in RFC6532.
      • used in authserv-id, reasonspec and pvalue part of propspec
  • Comparison of authserv-id should be done after converting A-labels into U-labels (see section 5.)

[lieser]

Currently looking into the added Internationalized Email support. If anyone has any real world example e-mails, please send them to me via e-mail. Thanks.

[lieser]

The a property in an ARH is now recognized and handled (i.e. check if the weak rsa-sha1 algorithm is used). The s property is still ignored, as it is also currently not used in the DKIM verification besides for the retrieval of the DKIM key.

I however decided to keep proper support for Internationalized Email out of the 4.2.0 release. Some partial support is now included and can be enabled with a hidden setting. Please let me know if anyone encounters a real world example (and best provide it to me). This certainly would increase to me the priority of implementing proper support for Internationalized Email.