Hello @MargoDawn ,
I'd recommend checking out this article that covers what suspended bounces are. If you decide to unsuspend an address from within your account, as a follow-up I'd recommend doing a Quicksend of the last email they were supposed to receive. If we receive a report from their domain / ISP about them being 'non-existent' again, then they'll likely be put back into suspension. You can check this by going to the contact's engagement history right after doing the Quicksend.
If the non-existent bounce occurs again, then it may be worth reaching out to our Deliverability team to see if they have any additional insight regarding that domain / ISP.