Before you begin sending resumes electronically, you must first find out the format the employer can and will accept your resume. Many employers are hesitant to open attachments because of possible computer viruses. There is also the headache of equipment or software incompatibility - for example, having to convert MAC documents to PC documents and vice versa. Read the job posting carefully to determine the format the employer would like to receive your resume.
If the employer will accept attachments, the body of your e-mail should be your cover letter, and you should send your resume in an attached MS Word file (the most universal format). If the employer will not accept attachments, you must paste the resume into the body of your e-mail.
Regardless of whether you send your resume as an attachment or as part of the body of the e-mail, you should send it as a plain-text document. You will need to create a "scannable resume" which will serve your purpose, whether or not the company utilizes document scanning technology. Your cover letter and resume should not be sent to the employer as separate messages; they should go as one unified e-mail message.