Blacklisted Public IP

My organization has two external IPs for outbound traffic. One of them, at some point, got put on a blacklist. However, everywhere I look (all the published black list sites) I can’t find which list it is. However,, and a few others will not let us send them traffic. Our users can not visit those sites if they exit the blacklisted IP. Does anyone know how we might go about fixing that? I’m at a loss. Thanks for any ideas. Changing that external IP isn’t an option as it’s whitelisted for other purposes.