Email providers keep a blacklist of email addresses and IP addresses that are known for sending spam. From time to time, you may find yourself on a blacklist.
There are a number of ways this can happen, however the most common is when one website on your host has been hacked. This happened to me in the past. A hacker placed a malicious script on one of my websites that was using an old version of WordPress. They then used my host's mail server to send thousands of spam emails. This placed my IP address on a number of known blacklists as a spammer.
Email providers are aware that this happens. That is why they allow you to remove your IP address if it has been placed on a list. They usually do not remove your IP from a blacklist unless you request it to be removed.
It is therefore up to you to monitor the status of your IP addresses frequently. You can do this through blacklist checking services. They will advise what blacklist services your IP has, or has not, been listed on.
In this page, I have listed many blacklist checking services that will verify whether your IP address has been placed on any blacklists.