Try removing and re-adding your outlook profile through the mail icon in the control panel. If this is a POP3 account try running scanpst to find any corruption. If that does not fix it try an uninstall/reinstall or a repair of office. If this is an exchange account and you are running on cached mode, try taking it off of cached mode and see if it works. If it does delete your .ost file and re-open outlook, re-enable cached mode and it will build a new one.