Why do IP addresses get blacklisted?
What is an IP address?
An IP address, or Internet Protocol Address, is a computer’s unique identification number on the internet. Theoretically speaking, every device connected to any type of network (wired or wireless) has an IP address. The specific role of the IP address is to identify and locate a device connected to a network and give it access to other devices on that network.
How do IP addresses get blacklisted?
While there is no reason to panic, it is good to be aware of the situation and know what to do if it does happen. Blacklisting an IP address can happen for many reasons ranging from malware detection to spamming.
Once an IP is blacklisted, it may cause email deliverability issues and bounce backs until the IP is delisted.
A web hosting package on a shared server can expose you to the noisy neighbour effect. The shared web hosting environment is a multi-tenant one, i.e. one architecture hosts multiple clients. When another customer unknowingly or intentionally sends an email that is later reported as spam, his or her IP address is blacklisted, which causes issues for other customers who share that same infrastructure. In essence, another user can potentially get the server IP blacklisted which can then affect you.
A bounce occurs when a mail server fails to send a message to another server, typically resulting in an automated email response. This will happen for messages originating from a blacklisted server. In the bounce, there will usually be reason codes that include the reason for the bounce, as well as information such as a postmaster URL.
The following are some examples of actual bounce messages indicating policy blocks. A bounce message can either be a hard bounce (5xx code) or a soft bounce (4xx code):
smtp;554 Message not allowed - [PH01] Email not accepted for policy reasons. Please visit http://postmaster.yahoo.com/errors/postmaster-27.html 
smtp;421, "4.7.0", Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily blocked.
smtp;550 SC-001 (BAY004-MC4F3) Unfortunately, messages from IP 126.96.36.199 weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors
What to do if your message is blocked or bounced?
It’s important that you contact the hosting provider and the company that blocked your IP address (as per the bounce) before doing anything else. Repeatedly emailing the blocked recipient may cause more issues.
You can check if your domain or IP is blacklisted here MXToolbox IP Blacklist Check