My mailserver is very German. When your mailserver tries to send a message, it does a reverse lookup on the IP address. If that doesn't deliver a valid hostname, you're out. But we are not done yet. If it gets a valid hostname, it does an A (IPv4) or AAAA (IPv6&) lookup on that hostname. And if it doesn't deliver back the same IP address, you are still out. It is fascinating to observe how often that uncovers that even big names get their DNS wrong. Hello, Spamcop ;)
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Obviously that's just the start of my German Mail Server vetting you. Blocklist, SPF, DKIM, DMARC checks follow. And I had to make some allowlist exceptions for mail providers whose emails I begrudgingly accept, even though they messed up their config. For a geeky deep dive, my blog series on all of that starts here: https://jan.wildeboer.net/2022/08/Email-0-The-Journey-2022/
Jan Wildeboer's Blog
E-Mail Done My Way, Part 0 - The journey
Warning. This whole series is not a simple HOWTO. This series is about how I run my mail server. I go through the configuration, line by line and explain. It’s not going to be a simple Copy/Paste to run your own mail server. You have been warned.
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