What to do when you get "POP3 Error 'Err Command is not valid in this state"

When attempting to connect to an Exchange 2007 using the POP3 protocol you receive the error” Err Command is not valid in this state”.

In testing, you telnet to the Client Access Server on port 110. You pass the appropriate credentials and password and still receive the same error.

Issue:
The application that is attempting to logon or also using the above example, the logon is being performed via Plain Text and by default Exchange 2007 POP3 will not allow a Plain Text logon.

Resolution:
Open the Exchange Management Shell and enter the following cmdlt:Set-PopSettings -LoginType PlainTextLoginRestart the POP3 service.