After rechecking your account, I found out that it indeed reached its sending limit. Since you have an active subscription with Windows Live Hotmail Plus, I created a back-end fix for so that you'll not be prompted by that error message again.
To know all about Hotmail's sending limits, visit:
http://windowslivehelp.com/solutions/sendlimits/archive/2008/12/22/hotmail-sending-limits.aspx