When writing tests I use the example.com domain for any URLs, email addresses, and similar. The reason: if my test unintentionally made a real request, or sent a real email, then it would resolve to a special-use domain. The Internet Assigned Numbers Authority (IANA) reserves the example.com domain for documentation purposes. Better that the request resolves there than to one that’s privately owned, such as test.com.
While reviewing some work recently I came across a comment from Ben about his suggestion to use the .invalid top-level domain. I learned that DNS prohibits installing certain TLDs like .example, .invalid, .localhost, and .test. Using test data such as [email protected] is safer than [email protected] because requests should never even reach DNS servers. While all four TLDs receive special handling, .invalid is the most non-resolving. For the technical differences between them, see sections 6.2-6.5 of RFC 6761.