Some mailservers send out of office replies and NDRs from a blank sender address. SMTP therefore has to accept this. Email with blank senders that are not from your network OR Out Of Office replies or NDRs can be blocked as spam
This rule identifies email where the displayed From name tries to hide the actual email address using a displayed name of an email address that differs from the actual email address
such as "firstname.lastname@example.org" <email@example.com>
In some mail clients only display name part of the From address or reply to field is displayed. This can lead to confusion for users if they think the displayed name is correct but in fact hiding
a different email address. For example a spammer may sender an email from "Your CEO" . Use this setting to force the actual reply to and/or SMTP envelope sender address to be published in the displayed part of the from field of received email.
So for example the above becomes "Your CEO " making it clear that the "Your CEO" was spoofed.
SMTP vs MIME
This rule checks the SMTP envelope Sender matches the MIME From field.