Feel free to contact our email delivery team at firstname.lastname@example.org, or contact them through Support, to get help with any of this process. We do not allow you to send bulk email to anyone who has not specifically opted in to receive the email from you. Depending on the age of your list, it may be better to approach this from a different angle. It is better to ask prior to spending money cleaning up a list you may not be able to use!
If our postmaster determines you can use the list, we do recommend cleaning the list as much as possible first. You can start by running the list through a service like BrightVerify to make sure you aren't importing any invalid addresses. Some email services will convert abandoned email addresses to "spam traps" and label you a spammer if you send to those old email addresses.
If your list is too old and cannot be emailed, you still have other options. You can import it with the "Bulk Email Status" field set to "opt-out" and market to them via Facebook Custom Audiences, Post Cards, etc. to encourage them to opt in again.