Even in an age of email and texting, hard-copy business letters have their place. Shifting from digital to hard-copy mode can be a challenge: An email can sound casual, but a written letter requires a degree of dignity and class. Don't worry about trying to sound original -- you're safer sticking to the tried-and-true formats and the rules of business communication etiquette.
Whether you're replying to criticism or request for your CV, your letter should stay on point. Never go beyond one page and never be discourteous. If someone has written you a letter of complaint seething with invective, stay polite and calm when you write back. If the original letter is about a business opportunity, say thank you. Never imply that by writing back you've resolved everything -- that the complainer should be fully satisfied, that you know you're getting the job. That's for your correspondent to decide.
The standard format for a business letter is single-spaced, with one space between paragraphs, and everything justified to the left margin. In the 21st century, it's acceptable to move away from the block standard, for example by indenting the paragraphs. Times New Roman, point-size 12, is almost always an acceptable, readable font. If your company has any preferences -- say, for instance, it favors indented paragraphs -- format it that way.
The standard layout starts with your business address. If you're writing on letterhead stationery, you can skip that. Below the address comes the date, then the recipient's name, business and address. Skip a line, and then address the recipient as "Dear Ms." or "Mr." unless he has a title such as "Dr." Don't use first names unless the initial letter writer did.
Wrap up the letter with a "Sincerely," followed by followed by your signature, then your typed name. If you're sending anything with the letter, such as a form, a cost estimate or your CV, write "enclosure" under your name, followed by a list of the attachments. If you're sending something in a separate letter, add a note such as "under separate cover: financial statement." A "cc" followed by a list of names tells the recipient you've sent copies to someone else.