Adding new contact - This is not a valid email address.

A member asked to update her email address to a different one. The new email address is a gmail account. When I tried to update her contact information, I received the error 'This is not a valid email address.' 

I've confirmed with her that this is the correct email. I made sure there are no spaces before or after the email address. Please let me know how I can proceed with this issue. I'm not sure what constitutes an invalid email address. Thanks!