Mail bombing is a specific type of denial-of-service (DoS) attack where an attacker sends a massive volume of emails to a single email address or domain. The primary intent is to overwhelm the mail server's resources (e.g., storage, processing power) or completely fill the victim's mailbox storage quota. This prevents the target from receiving legitimate email and can render the email service unusable. The attack's definition directly matches the scenario described in the question, focusing on volume to cause a service outage.

A. Email spamming: This involves sending unsolicited bulk messages to numerous recipients, typically for commercial or malicious advertisement, not a targeted DoS attack on one system.

C. Phishing: This is a form of social engineering that uses deceptive emails to trick users into revealing sensitive information, not to overwhelm a server.

D. Email spoofing: This is a technique of forging the sender's address in an email, often used in phishing or spam, but it is not the volume-based attack itself.

1. National Institute of Standards and Technology (NIST). (n.d.). Mail Bomb. In CSRC Glossary. Retrieved from https://csrc.nist.gov/glossary/term/mailbomb. The definition states: "An attack in which a large quantity of messages are sent to a user or system to try and cause a denial of service."

2. Stallings, W., & Brown, L. (2018). Computer Security: Principles and Practice (4th ed.). Pearson. In Chapter 7, "Denial-of-Service Attacks," the text distinguishes between spam and targeted DoS attacks like email bombing, which aims to overwhelm a specific target.

3. Schuba, C. L., Krsul, I. V., Kuhn, M. G., Spafford, E. H., Sundaram, A., & Zamboni, D. (1997). Analysis of a denial of service attack on TCP. Proceedings of the 1997 IEEE Symposium on Security and Privacy, 208-223. https://doi.org/10.1109/SECPRI.1997.601338. This foundational paper discusses various DoS mechanisms, including resource exhaustion attacks at the application layer, of which mail bombing is a classic example.