Client doesn't want to use Google for email. Client complains about email going into spam. Enki pulls hair out on a regular basis explaining to client that this is what he gets for having a small email server.

You can follow the emails spec to the T, have everything from DKIM and SPF and DMARC, set to strict. You can have a BIMI record. (I think I'm recalling that acronym correctly.) You can have an IP address that is relatively clean with a decent history of not sending spam.

And you will still get shunted into spam because SMTP is a completely captured protocol at this point. I run more than one email server for myself and others. I've had good reputation following the spec as well as I can. And no matter what, I'm almost always in spam.