This article will explain you the reason for mail servers to get blacklisted.
What is a blacklist?
Depending on who you ask, most people who attempt to track the global level of spam being sent, estimate anywhere between 80%-95% of all email transmitted daily on the Internet is spam.
That means that simply relying on manually going through all of your own email, is not the most efficient method for weeding through spam messages anymore these days.
As such there are published public blacklists of mail servers that have been relaying spam. This allows other mail servers to first check if they're receiving an email from a server's IP address that might have possibly been flagged for sending spam in the past.
Why is my server on a blacklist?
It's not very uncommon for a server's mail IP address to temporarily end up on a public blacklist, especially if you're on a shared server. It could be because of the overall volume of mail that is coming from that server, messages seem to have characteristics of spam in them, or a few other factors as well.
One common cause for getting blacklisted can be with the use of email forwarders. For instance, if you have the user email@example.com, and you're forwarding all of your email to a firstname.lastname@example.org account. If someone spams your email@example.com address, our mail server could then attempt to forward that onto Gmail, and their servers might see our mail IP address as relaying the spam message to their server, even though it wasn't the originating server of the spam.