Résumés sent by e-mail need a brief cover letter. Use two or three quick paragraphs with three to five sentences telling your reader where you heard about the position and why your qualifications are a perfect fit for the position's requirements. E-mail is intended to be short, sweet, and to the point. The cover letter will give the employer a brief, but to the point, synopsis of who you are and what you can offer. If an employer is sifting through hundreds of applicants, he or she is going to easily narrow down the choices based on what the cover letter says.
Every résumé sent by mail or fax needs a personalized cover letter even if the advertisement didn't request a cover letter. Even if an employer doesn't specifically ask for a cover letter, it is important to include one. Many employers have been known to use this small thing as a test that weeds out the unprofessional applicants. All professionals automatically know that a cover letter is critically important and worth the extra time invested.