IP addresses of sending email servers can be added to the blacklist if they are identified as sources of spam or other unwanted activities. If the sending server is used for mass spam distribution, it can impact the entire IP.

Sending domains can also be added to the blacklist. If a domain is associated with sending spam, phishing, or other forms of unwanted activity, email service providers may choose to block or mark emails from that domain as suspicious.

In practice, many security systems and anti-spam services use both IP and domain blacklists to enhance the effectiveness of filtering unwanted online activities. Ultimately, whether an IP address is added to the blacklist depends on the policies and criteria of the specific blacklist, as well as the actions taken by system administrators or internet service providers.

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