The unfortunate experience of having your site blacklisted can cost you money, reputation and time spent finding and fixing the offending site behavior. When you’re blacklisted, potential visitors are alerted that your site has been blacklisted, and they’re advised to back out. No good for business.
You can also be blacklisted by having your emails get rejected. Same applies: bad for business.
If you’re innocent of malicious behavior but you’ve been blacklisted through no fault of your own, you probably got hacked. If you didn’t get hacked, it could be your web host’s email server.
No worries, it happens everyday and there’s a way out.