3.1.5 Message Processing Events and Sequencing Rules
The NTLM POP3 Extension is driven by a series of message exchanges between a POP3 server and a POP3 client. The rules that govern the sequencing of commands and the internal states of the client and server are defined by a combination of [RFC1734] and [MS-NLMP]. Section 3.1.1 defines how the rules specified in [RFC1734] and [MS-NLMP] govern POP3 authentication.
If the client receives a message that is not expected for its current state, the client MUST cancel the authentication process and transition to the completed_authentication state.