Account Name blank on one account only

I'm using WLM for five different e-mail accounts. Four are POP and one is
IMAP. It all works fine with one small exception.

The main mail page is generally left on 'Quick View' so that all incoming
mail shows in chronological order. One of the 'Columns' across the top is
set to show the 'Account' so that I can identify whether the mail is
personal, business, etc. by the account name. All the account names show in
this column just as they are listed per:
Tools/Accounts/Properties/General/Mail Account except for one, My @msn.com
(Hotmail) account. Incoming mail for this account has a blank in the
'Account' column. Is there a reason why?