> I.e. they stay on top of and evolve with best practices over time so you don't have to.
> Is that in any way accurate?
For the most part, yes.
It's all about delivering as much information as possible to aid the receiver with the spam detection process. The goal is to reduce false positives/negatives. The beauty of this is of course that if you are in fact sending spam, this will work against you.
Mailchimp does this very well, they actually have people working for them with 'email deliverability expert' as their job title.
The problem is that rolling a simple email sending solution is trivial to set up, but hard to get right. It takes surprisingly significant engineering resources to build a solution that complies with all the relevant RFCs, scales nicely and follows new developments. Resources that are probably much better spend elsewhere.
My job is helping customers optimise their email infrastructure [0], I have analysed hundreds of domains, and my observation is that the vast majority of home-grown email solutions lack even the basic provisions for deliverability.
My advice: don't roll your own email solution, unless you can afford full-time engineers to do so. There certainly is a tipping point (as always) where building your own solution is more cost effective as a SaaS solution, but that is certainly not at 500€/Year.
[0] Disclaimer: I'm the founder of Mailhardener