That type of problem can be caused by interference from a non-
compatible antivirus program. Which antivirus are you running?
As a minimum, email scanning in the antivirus must be turned off,
although that may not be sufficient to eliminate all bad effects.
In a worst case scenario, your antivirus may need to be uninstalled.
As a possible quick fix, delete that account, restart Windows Mail,
then recreate the account. Accounts can get corrupted by antivirus
email scanning.
If no improvement, consider upgrading to Windows Live Mail,
because it is less prone to suffering bad effects from overly intrusive
antivirus programs:
http://get.live.com/wlmail/overview
Email scanning in any antivirus must be disabled, for reasons
explained here:
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm
