After configuring my Email Router Configuration Manager for Incoming Emails on POP3 mailbox, when I tested the access with any of the users then the following error showed up.
Incoming Status: Failure – An error occured while executing POP3 command “<Command removed for security reasons>”. The server response was: “-ERR not supported”
This was due to the reason that POP3 does not use NTLM authentication. Instead, POP3 uses Clear Text.
On changing the Incoming email profile using POP3 to use Clear Text Authentication instead of NTLM authentication, the error was resolved.