It happens sometimes, when I sign up for a forum or a website that I get told my e-mail address is invalid.
It looks like this:
Is there any reason why this might be considered invalid?
It's probably the underscore before the @ sign. It's not technically invalid, but it's not especially common, and I can easily see that badly written format checkers would flag it as invalid (you're not allowed to have a full stop before the @, so they might generalise that to all punctuation characters).