Viva.com, one of Europe's largest payment processors, sends verification emails without a Message-ID header — a requirement of RFC 5322 since 2008. Google Workspace rejects them outright. Their support team's response to my detailed bug report: your account has a verified email, so there's no problem.
Email address regex: ‘.+.+’
Joke, here’s some reading on it. Basically, E-Mail address can have almost any form and you better use a library that doesn’t use regexp parsing.