Spammers

Deliverability is an issue in both directions:

- when you send emails from MyWebsite.com, you want to make sure that Hotmail, Yahoo, Gmail, Proton Mail, etc users get your email.

- when you receive emails to your hi@mywebsite.com email, you don't want spam

V1 of email providers ostensibly solved this problem using lots of data as well as "supervised" machine learning (e.g. you telling the provider that a specific email is spam).

Self-hosting seems awesome if you're only handing out the email to a few people (probably a few hundred max?), you don't think it will be widely shared, and it's a slightly unique email that makes it hard for a script to guess (e.g. "hello@mywebsite.com" will be easily guessed but "john2@mywebsite.com" may not be)

- because google would prefer you use gmail so they can train their data models on you

- google would also prefer to have a monopoly on all email so that g-suite is necessary to reach your customers.

- because your IP address was at one point an e-mail spam server, but, that only really matters because of the above two reasons. Any nit, however irrelevant, will be used used to beat you into using gmail.

All anti-spam systems have non-zero false positive rate. It is hard to fix all FP in advance, but relatively easy to do this on case-by-case basis. When an FP affects some large sender it gets fixed. When it affects small senders - it is ignored because Big Corps don't care about small fish. They would be happy if all small mail servers will disappear and the market will be controlled by a few big companies.