The PIPELINING capability indicates the POP3 server is capable of accepting multiple commands at a time. (A similar capability is defined for SMTP servers.) Mail clients do not have to wait for a response to a command before issuing a subsequent command.

Pipelining breaks the command-response sequence of the protocol, which (in our opinion) is undesirable. Its quoted benefit (grouping multiple commands and sending them as a unit) has a limited value in a small network environment and is outweighed by an increased network vulnerability to threats.

Consider for example an attacker sending thousands of commands or responses through a PIPELINING mail proxy in a denial-of-service attack. Assuming they are formally valid (even if invalid in the context, such as deleting nonexistent mail messages), the pipelining proxy will pass them through and flood the network.

This is often the case with spam that is sent by specialized spamware that blasts mail messages as fast as possible without waiting for responses or checking for errors rather that by normal mail software.