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example.com and example.org were specifically reserved for testing: https://www.iana.org/domains/reserved

I've seen sites that specifically deny @example.com email addresses.

Which is perfectly reasonable. They're looking for an email address that they can reasonably expect to deliver mail to, not just an address that is technically valid.

This sometimes gives you special/admin access. Good if you are security testing something, bad if you accidentally hack a site.

I bet the guy who owns test.com gets a lot of interesting junk.

or just me@example.com