RFC 822 - sgml/signature GitHub Wiki
- https://devblogs.microsoft.com/scripting/use-powershell-to-parse-email-message-headerspart-1/
- https://www.clevercomponents.com/products/inetsuite/messageparser.asp
- https://wiki.tcl-lang.org/page/Reading+and+parsing+RFC+822+headers
- https://practical-scheme.net/gauche/man/gauche-refe/RFC822-message-parsing.html
- https://datatracker.ietf.org/doc/html/rfc6376#section-8.2
- https://ocaml.org/p/mrmime/0.4.0
- https://toolbox.googleapps.com/apps/dig/#TXT/
- https://support.google.com/mail/answer/29436#Gmail&zippy=%2Cgmail
- https://support.google.com/a/answer/11612790