Peter N. M. Hansteen asks the question, "Does Your Email Provider Know What A "Joejob" Is?" in his blog and provides some data and discussion. He provides anecdotal evidence which seems to indicate that Google and possibly other mail service providers are either quite ignorant of history when it comes to email and spam, or are applying unsavory tactics to capture market dominance.
[Ed Note: I had to look up "joe job" to find out what it is. According to wikipedia:
A joe job is a spamming technique that sends out unsolicited e-mails using spoofed sender data. Early joe jobs aimed at tarnishing the reputation of the apparent sender or inducing the recipients to take action against them (see also e-mail spoofing), but they are now typically used by commercial spammers to conceal the true origin of their messages.
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(Score: 0) by Anonymous Coward on Monday April 25 2016, @10:53AM
Yep, they don't care. My employer runs their own email server, and we have constant headaches with emailed reports we send out being sent to the spam folder or sometimes just disappeared entirely.
My standard response has been to forward the copy of the email (report engine saves this) and just say something to the effect of let your IT department or consultant know. If they want an escalation, the server admin will forward info from the mailserver log showing that we did our part and delivered it "to the next relay."
Of course, nobody ever does anything about it to resolve the problem. I suspect there's a lot of apathy out there among small businesses that use gmail in particular. Everybody knows that if Google doesn't want to do something, Google doesn't want to do something, and they'll tell you "fuck you" for trying to contact them about a problem.
I forget if we've got DMARC set up, but SPF and DKIM are there. Doesn't make a damn difference.