If you only use Scan feature without using Mail Inbox Checker feature then this issue may arise rarely. The main reason behind this is that email server where it connecting to verify that email name address is a public email address server that our script in built smtp scan not supporting / or server protected over the firewall and can only be valid for MX destinations. You can checklist of unsupported email servers from here. But they will be supported gradually in our validation scanner system as we implement updates in the future. Alternative you can use second level validation Mail Inbox Checker on top of default scan and you will get 100% accurate response data.
Mail Inbox Checker feature is an optional feature which sends direct email using dummy stored templates to validating list. It requires an external SMTP & IMAP mailbox connections to do this work who allows bulk email sending with no limits.