Marketo basically requires the email field in order to create/maintain a lead record - since, at heart, Marketo is still an email marketing/nurturing system. When you have an invalid email, I highly recommend you mark it as "email invalid" in Marketo, and it will no longer get your email blasts. You can then use other campaigns to check if the email address is updated, and then mark the lead as no longer 'email invalid' if the email field changes.