Using real domains (like yahoo.com) for "fake" e-mail address is not a good practice. There are domains like example.com, example.net or example.org for such purposes (RFC 2606, RFC 6761).
That depends on what you’re trying to do. If you want it to be an obvious example, @example.com is great. If you want it to look like real data, such as in a demo or a screenshot, it’s not so great.
It’s a bit like saying that every demo user should be called John Doe or Jane Doe if they aren’t actual people.
