Whether you're sending your cover letter as hardcopy or email, begin your message with a greeting.
Include the recipient's name in your greeting, if possible. Don't start out with "Dear sir or madam" or "To whom it may concern." If you don't know the person's name and have no way to find out that information, begin your email with a greeting that includes a logical professional title, such as:
Dear Hiring Manager,
Dear Human Resources Manager,
Dear Personnel Manager,
Another possiblity is to start by writing simply "Hello."