I know that you're making a joke about rejecting legitimate email from small senders, but if you wanted something to hit spam, sending from a VPS where the SPF wouldn't pass would be a decent way (though it might just get rejected entirely).
For those who don't know, SPF allows one to specify which IP addresses can send mail for a domain. Your VPS likely wouldn't match the range for whatever domain you're trying to send from.
Careful with this, multiple failed SPFs might lead to an abuse report. As others have said I've found even with SPF/DMARC and everything set up correctly it still goes into spam so you don't even do need anything special!
I've read so much conflicting information only whether it's even feasible to run your own mailserver that I had to try it.
Turns out I'm one of the lucky few (or silent majority) that had no problems whatsoever after getting my newly acquired ip off from a few lists. Google recieved my mail first try without even looking into spamlists.
...Maybe it helps that my domain is a relatively expensive one?
I think the challenge is keeping it up for years. And issues like you thinking that the mail is sent, only to learn later that a spam filter ate it somewhere. Now you have to think: what else is being eaten? For how long? What if you do need to send the email right now because of a deadline or similar?
So currently it's not infeasible, just an uphill battle.
I had no issues with Google's mail servers, however, Microsoft's let a few of my mails just vanish. They were accepted with a 200 OK but didn't even go into the spam folder. Granted, I'm using an .xyz domain, but still, I think this is hostile behavior.
Another fun story, I once tried to contact a public authority and my mail was rejected. I then tried to contact the postmaster inbox, same thing. I had to use my gmail account and was initially met with a dismissive reply. Only after asking what the job of the postmaster incorporates if not helping users with delivery issues and whom else to contact I got helped reluctantly.
