Although this is an evolving area of the law, you should assume that NONE of your emails, whether internal or sent/received from an external system, are private.  Your boss can read or monitor these emails.  Courts have consistently held that, because your employer owns the computer system at your place of employment, it can monitor how that system is being used.  Even when the system in question allows employees to mark emails as private or confidential, the resulting 'expectation of privacy' may be overcome by an employer's compelling need to review or read the emails, such as if the employer is trying to investigate a claim of sexual harassment.  Hope that helps.  Follow this link for a discussion of other employee rights in MA.