> Viva.com's outgoing verification emails lack a Message-ID header, a requirement that has been part of the Internet Message Format specification (RFC 5322) since 2008 > ... > `Message-ID` is one of the most basic required headers in email. Section 3.6. of the RFC in question ( https://www.rfc-editor.org/rfc/rfc5322.html ) says: +----------------+--------+------------+----------------------------+ | Field | Min | Max…
The official definition of SHOULD per RFC2119: 3. SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course. Not sure how the people at Google interpreted this about the message-id
Anyway, in general you can expect that doing unusual but technically valid things with email headers will very often get your messages rejected or filtered as spam.