Well I see Lisa's issue my be more of a technical issue rather than a business issue. If it's more of the latter one option is to validate the email on the Marketo form. There are many email validation services that have web service APIs that can in validate the email address before allowing the user to submit the form. Depending on the offer and industry it may be a bit aggressive.
Another apporach could be to validate the email in the background after capturing the data in Marketo so that you suppress them systemically from your email campaigns, this help keep your deliverability and overall sender reputation as high as possible.