Guidelines and considerations for creating your cover letter for submission via email:
Keep it very brief. Summarize your strong points, and make the employer see you as a good candidate for the position before they've even read the rest of your resume.
Because you are emailing your cover letter and resume, you have an opportunity to catch the employer's attention by using the "Subject" line creatively. Unless otherwise specified, don't just use the job id reference number, but the title of the job and a reference to your experience, such as "Freelance Writer for Website Content Contributor Position".
Don't forget to include keywords, because your resume will likely be entered into the company's database, and will be found when the staffing manager performs a search using keywords for the position.
As with your resume, your cover letter should be submitted in ASCII format. Just create your cover letter in your word processing program, and save it as a "plain text" file (.txt). This removes all formatting that the word processing program has applied, and allows the email program to wrap the sentences, and keeps your cover letter from looking garbled and basically unreadable.
Use standard protocol for creating your cover letter. In other words, you will still include your "Dear xxxx" line, skip a line and begin a paragraph. Everything should be left aligned, though, and do not use justified alignment.
Conclude your cover letter by closing with "Sincerely" or "Cordially", and sign (type) your full name, just as you would on a paper cover letter.
Last, make sure you use the spell check tool and correct mistakes before you send your cover letter and resume. Spelling errors can get your email canned right away!