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Why would it need to be designed? Email delivery depends on DNS, which is unencrypted and spoofable. Spoofing emails is also doable.

If the dns lookup on a com1000 domain fails, the email won’t go anywhere.

Correct. But if a DNS response for that domain is spoofed. It will.

DNS is a very old protocol that still has lots of problems and mitigations like DNSSEC are only partially deployed.

Email is an inherently insecure medium. If an attacker can compromise the DNS responses for your mail server, you're hosed.