Email Rejected by DNSBL

DNSBL is a blocking list that keeps a record of specific IP address/network with spamming history.
It is widely used as one of the antispam mechanisms on mail servers.
There are many DNSBLs available on the internet with their own criteria to list and delist IP address.

Sender's IP address may get blacklisted if there is abusive activity going on their server or network, such as spamming.
The spamming may not necessarily comes from the sender's domain/account, but he can be affected by it, as he is sharing the same IP address with the abuser.

Most of the time, mails are rejected on the server if the sender's IP address is blacklisted by two or more DNSBLs.
If this happens, the sender is recommended to report this issue to his mail administrator so that they can stop the abusive activity permanently and apply for IP address delisting from respective DNSBL involved.

Free mail service providers users such as Yahoo, Gmail, 163.com and others are often affected if their IP address were blacklisted.
This is because spammers may easily sign up for new email account and blast out the spam mails from their servers.
However, these providers have multiple mail servers with a wide range of IP addresses.
Therefore, each time their users send out mails from their server, they will randomly choose any available server.
This way, each email transaction may come from different IP address, hence it may not be rejected if it originates from IP address with good reputation.