We recommend that the cover letter you send be the actual email itself. Do not send an email and attach both your cover letter and resume.
Just because it is an email does not mean it's not professional. Remember to follow the rules of professional correspondence using correct grammar and punctuation. Our recommendation is to write the email text outside of your email program - spell check, etc. and when it is perfectcopy and paste it into the body of the email. Next you'll add the subject line which is likely the position to which you are applyng and your last name and at the very end - address the email.
Attach your resume, preferably as a .pdf file. Open it after attaching to verify that it is the document you intend. Once you hit the "send" button - it'll be too late!
Don’t fold and stuff your resume in an envelope (even the nice ones you can buy to match your resume paper)!
Buy envelopes that are the same size as your resume and slip your cover letter on top, then your resume. Type an address label and return address label (or stamp if you have it) and mail flat.