That type of problem is usually 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.
Email scanning in any antivirus must be disabled, for reasons
Why you don't need your anti-virus to scan your email - A Cloudeight Information Avenune Tutorial
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: