Does anyone know any good free SMTP services that work in 2020, and how to set it up?
No, there are none. Email spam is the problem, so it’s pay to play.
Email is very inexpensive compared to other costs associated with Discourse; why are you looking for free?
I ask, because everyone is different, and I’m curious as to why folks approach Discourse installation like this. Often it has to do with payment systems involved, them being inaccessible. Why do you want a free SMTP service? 
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Just use mailinabox on a $5 droplet or similar. It’s painless and you don’t have to worry about it hitting spam
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Mailgun is still (again) free, but only for very small volumes. More precisely, it’s a paid service, but they don’t bill you if your usage is less than $1 in a given month.
I currently use MailJet’s free plan which gives you 6k emails/month. Their paid tiers are very reasonable too.
(Sorry @danb35 I meant to reply directly to @CorruptBlu)
Ray
I use sendgrid and it’s free. One hundred emails per day and the first 40,000 in a month are free.
Cool, lot’s of entry tiers and a self-hosting option. Still, depending on a person’s location and means, they may not be able to access those services due to financial or identity system incompatibility.