I'll make a stab in the dark, since you failed to include the complete
error message.
"Unexpectedly terminated" is usually caused by interference from an
antivirus program. Which antivirus are you running?
As a minimum, email scanning in the antivirus should 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.
Email scanning in any antivirus should be disabled, for reasons
explained here:
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm
As a possible temporary fix, delete that corrupted mail account,
restart Windows Mail, then recreate the account.
If no improvement after disabling email scanning, consider upgrading
to Windows Live Mail, because it is less prone to suffering bad effects
from overly intrusive antivirus programs:
http://download.live.com/wlmail
