alt Hacker News> - 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).
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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.
I use a reputable email provider (Mailbox.org) and my emails often end up in the spam of Gmail users.