> - Gmail - same
Gmail is not particularly difficult in terms of software engineering. The moat with email servers is IP address reputation.
I could, today, install Postfix SMTP and some IMAP as well, and watch my email all go to spam directly, if delivered at all (ISP might block them).
I use a reputable email provider (Mailbox.org) and my emails often end up in the spam of Gmail users.
FWIW, I've done exactly this + DKIM & DMARC on a residential IP and was able to deliver to Gmail & Protonmail with no problem, but didn't test it long term. Same with a server on AWS. I'm not sure if self hosting email is an overblown problem or I've just been very lucky somehow. Obviously it's still a big issue if for ex. Outlook's spam filters happened to be slightly more sensitive and dropped my mail; I never encountered issues but eventually I just used a custom domain via Proton for peace of mind.
Not so bad for a single person. But once you start being an email provider, you get bad actors using your service to send spam emails. Which can cause all of your sending ips to blacklisted at once.