NTLM authentication has been a standard authentication that was designed by Microsoft and built into Windows. It is widely adapted by a variety of software (including Windows) as a means of authentication. The authentication protocol was also supported for staff that used the MSN Chat Admin Client to connect to and administer MSN Chat, however according to User:Koach NTLM Clear-text authentication was the favoured method of logging into the Chat Servers.
5.2.5. Example Client: AUTH NTLM I : Server: AUTH NTLM S : Client: AUTH NTLM S : Server: AUTH NTLM * userid@domain 03FA4534C