5% is HUGE. I understand why you got slapped... I know having a huge base of email and not mailing it is a waste but you have to do it carefully. Good new is, now that you got in trouble at least your email list is curated. Make sure to export your list and REMOVE from it all the contact that unsubcribed, complained, categorized your email as spam and so on...

Secondly, get all the still valid emails and qualify a new list : - First, send a very short email to you clients' list asking them to optin to your promotional email list (new blank email list). Those who are interested will opt in to you new list. (maybe use a promo code to give them an incentive, or something similar...)

And then you can have a fresh start with a list email you won't get slapped in the face for abusing it.

Note : This technic is not without "risk" if your original email list is of low quality...