If this has been going on for weeks, then do not do a system restore. Instead , what e-mail service are you using ? You may have incorrect settings in that e-mail settings, and it is not Windows Mail's fault .
For example, i have g-mail from google, and i have it set to forward to my comcast email , which is my Windows Mail by default settings i have set up with Comcast.
Check your settings out , and post back if we can assist . In fact, post back so other users can learn from this issue .