This. I've rattled on about this for days here. I'll still run one, but if you need emails to make it somewhere it's impossible to guarantee they make it to their destination.
BTW The rules (enki listed) you're expected to follow, Google, Yahoo, and Microsoft don't actually following either because they don't have to, they have reputation. They don't even have working rDNS which you're, by law, in the US required to have to send emails (among other things).
Yes, btw you need permission from the US government to be allowed to send email on US networks. Telcos can be legally responsible for your traffic, which is why most residential connections block outbound traffic with dst port of 25.
There is partially a way around it though. You can pay for an SMTP relay with a good reputation and they have a deal with Google and friends. resend.com supports this and is free for a small number of emails. It's another SaaS so use it at your own risk. But unless you know someone who can get you in the door, there is no option if you need to send emails to user's inboxes. I've also had good luck with Inmotion Hosting email services for commercial use.
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