Wow since you're so concerned and helpful, here's a tidbit: Everyone who has tried to run a personal mail server in past five years can't because of gmail.
N=1, but my small email server actually hosts emails for a business, and the only problems I've ever had were with Microsoft, and they went away after i jumped through some hoops for them. I'm using a linode and dkim/spf/dmarc/tls/rdns.
Personal email server will send ~100 emails a month (maybe 1000 if they are extra prolific). Roughly half of those emails will be in reply to emails that originated on your mail server (you can link those with basically unique message-ids that you generated). There will be a bunch of extra safety measures on the server (reverse DNS, SPF records, no open relay...).
Spammers, on the other hand, will send thousands of messages every hour. And have none of that "other" stuff.
Spammers can buy cheap VPS and setup all this stuff with scripts. So I don't think that it really helps. It's good to have it (to prevent spoofing), but I'm not even sure that Gmail will use wrong SPF record to deny e-mail, it's not really part of SMTP, those are just optional additions.
But volume is a good argument. I don't think that spamming could be cost-effective with something like 100 e-mails per day from a single IP-address. So why not just let small servers pass every filter in the world (may be except reverse-dns record) as long as they are small? The danger should not be very big.
Of course I might be missing something, I have no idea how spammers really operate. Or may be Google just don't care about personal servers at all.
