This is because input type email isn't in HTML4 specification. It's a specification from HTML5, but the validator work's on HTML4 standard and will show the error when validating forms with input type email.
Using input type="email" for email addresses is better than using type="text" because then users of (iPad/iPhone, Android) will have the correct keyboard come up for entering an email address. The keyboard will have the @ symbol and .com on it.
There will be no problems with any older browsers, they all treat type="email" as type="text" anyway.
So there is an advantage to using input type="email" for email addresses, Mobile Accessibility. The only disadvantage is the validator. Sometimes the validator falls on the wrong side of these kind of issues.