In case your host’s IP address is blacklisted, any email sent by you or your company will bounce back from the recipients email provider in case they are a part of the same blacklist. This can be highly intimidating for an individual user and even for a company that relies on email marketing to reach out to a huge number of customers, this can be very disappointing. 7. Blacklisted IP. Every time you go online your ISP will assign you a static or dynamic IP address which acts like your home address. If that IP has been used to send SPAM or perform DOS/DDOS (Distributed Denial of Service), webmasters would have blacklisted all such IPs abusing their services.