Although we cannot guarantee that you won't ever receive any bounce backs, if you only use the email addresses that are validated as OK there is a very minimal risk.
Addresses validated as OK will always be valid mail boxes, however issues may occur in the mail sending process. This can be caused by the target infrastructure or your email service provider.
Email addresses verified as 'unverifiable' can still be used in your campaign. However some of these email addresses may not exist which can cause an unwanted bounce back. One solution would be to use the OK addresses in your campaign and reintroduce a proportion of the unverifiable addresses into your email campaigns in small amounts. This will ensure bounce rates are kept to a minimum and you can still keep the contacts with dodgy mail boxes!