Have you been emailing the same list for months and months, or even years, without performing data updates? If you email invalid email addresses frequently, you will be blacklisted.
Email addresses go stale much quicker than telephone numbers or postal/physical addresses. If you have this type of customer information, make use of it. Relevant incentives are always a good way to motivate customers to update their details.
Best practice would be to use a web form to allow your customers to update personal information and preferences 24X7. Even if you have web forms where customers can update their details, you will still be receiving incorrect email addresses if you do not have data validation on the web form.