Comment by st_goliath

> 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 number | Notes                      |
    |                | number |            |                            |
    +----------------+--------+------------+----------------------------+
    |                |        |            |                            |
    |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/

                             ... bla bla bla ...

     /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
    | message-id     | 0*     | 1          | SHOULD be present - see    |
    |                |        |            | 3.6.4                      |
    |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/

                             ... more bla bla ...

     /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
    | optional-field | 0      | unlimited  |                            |
    +----------------+--------+------------+----------------------------+

and in section 3.6.4:

    ... every message SHOULD have a "Message-ID:" field.

That says SHOULD, not MUST, so how is it a requirement?