@Sonny 

I would not suggest starting immediately with a blast to whole list.. I would say probably it is best if you start with the engaged that you have now and gradually increase the sends.. 

So first you will be sending just to current engaged and then from time to time like maybe 1ce every 3 sends include part of the unengaged list. 

I don’t know the details but lets say that right now you have 5K engaged with 50% OR I would say that you can add up to 3K additional without hurting the deliverability much. So that would mean if you had 20K you have 15K that are not engaged now.. divide them in 5 smaller lists of 3K and then from time to time add one of those lists to the campaigns so they can become re-engaged again eventually..

Agree with Bobi!

I’d look back at the engagement prior to going cold on sending and segment it from there on top of your current engaged. After you establish good engagement rates with that core group, you can slowly bring in pods of the remaining inactive. Just recommend your segmentations take into account emailing someone multiple times during these re-heat and not opening. You don’t want to continue to sending to those truly cold subscribers, that will end up hurting your deliverability.