There were many threads previously on how bad/impractical it is to self-host your email. For example, this thread[1] is just a few days ago.
I was wondering if anyone had any experience combining the microsoft 365 business basic (6$ a month) with self-hosted email server? By relaying SMTP through the Microsoft provided outlook server, would my custom domain be free from being marked by spam?
[1]: https://news.ycombinator.com/item?id=31180379
Nowadays, I just use mailcow (https://github.com/mailcow/mailcow-dockerized) for the setup part and have a much more polished experience. Email deliverability is not a problem. Generally, you just have to make sure to correctly setup DKIM/SPF (and DMARC) and check if your IP is on some blacklist. You can get it removed easily. (Edit: Also required is forward-confirmed reverse DNS, see below).
There was one provider that denied incoming mails from me, even though I got the IP removed from every blacklist I could find. I wrote a short mail to the admin contact and got told I had to host a web page with contact information on the same IP. Since being whitelisted there, everything works like a charm, couldn't be happier.