just in case anyone is misinterpreting my previous post, I fixed my problem purely on the phone - I didn't go anywhere near the NM website, nor did I delete my actual NM account, I only deleted the NM setup on the phone.

All I did was to open the email client and from the "Email" drop down at the top, I chose "Edit Accounts", then selected "Nokia Messaging". I then accepted the "online editing .." warning by selecting "yes". Then at the "Settings for Nokia Messaging" screen I selected "delete" to remove the NM account from the phone. I didn't delete my NM accoutn from the NM website.

I then just set up NM again on the phone and my problem was gone. This may not help you, but it's what I did & it worked for me.