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?
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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).