I've had to set up someone's mail server last year. All mails sent to gmail were silently dropped until we set up all the current buzzwords for the domain/email server. Then they magically started to show up.
Possibly we were lucky that "just" setting up SPF DKIM etc fixed it.
SPF and DKIM are nowadays the very basic methods used to verify if your emails are spoofed or not. Not having them in place is as good as setting up a spam farm.
This used to be a cardinal sin for mailserver operators; mail should always either be delivered, or result in a non-delivery notification ("bounce"). Dropping mail on the floor was a no-no.
Then along came gmail...
They do: the test emails were never rejected, but never arrived in the test gmail account either. Not in spam, not in all mail, not in the inbox.
And that's fine because that's exactly what SPF/DKIM were designed to do. The spam folder is not a dump of true positives. It's the bucket where you train the filter by evaluating somewhat likely false positives.
A perfect technical configuration doesn't make you inherently trustworthy or provide you with faultless reputation.
Users' interests are served by being informed when this happens. And plain text correspondence is not easily mistaken for malware.
> A perfect technical configuration doesn't make you inherently trustworthy or provide you with faultless reputation.
No one claimed such.
That's exactly what it should be. If somebody sends me an email, it should arrive somewhere that I can see. If that means needing tiers of spam folders, each one spammier than the last, then so be it.