I see that the "/ni" username switch (disables the IDLE extension for the duration of the IMAP login session) was only added in v4.5.6.
I tested this with a client who is running v4.5.6 NE and it works - with the /ni switch after the domain; ie:
username@domain.com/ni.
I'll upgrade my server running v4.5.5 open source to v4.5.6 tonight and I'll report back if it doesn't work.
While it is good to get this working, the IMAP idle functionality is a key feature for phones/handhelds as it would provide push email.
It was one of the features I checked for in the IMAP client of the S60 3rd Edition software.