What type of attack might result in users being unable to access their email due to overloading a mail server?

The situation described involves users being unable to access their email because of excessive demand placed on a mail server. This aligns with a denial of service attack, which is specifically designed to disrupt the normal functioning of a service by overwhelming it with traffic or requests. In this scenario, the attacker's goal is to render the email service unavailable to legitimate users by flooding the server with more requests than it can handle.

This type of attack effectively prevents any legitimate communication or access to email, demonstrating its impact on availability, which is one of the core principles of information security. The denial of service attack does not require infiltrating the system or stealing information; its primary objective is to interrupt services for users. In the context of the other options, phishing attacks typically focus on tricking users into giving away sensitive information, man-in-the-middle attacks involve intercepting communications, and data leaks pertain to the unauthorized exposure of data, none of which directly lead to service unavailability in the same manner as a denial of service attack.