Formal business letters require a specific protocol no matter what the letter's intent. In most cases, a business letter should be succinct and brief, usually fitting onto one page. While you should use a clear, formal writing style, there are several cases in which using abbreviations is appropriate to save both space and time.
Both prefixes and honorifics are expected to be abbreviated in the salutation and elsewhere in the letter. For example, a business letter would start off with "Dear Mr. Johns," not "Dear Mister Johns." The plurals of all prefixes and honorifics are also abbreviated, such as "Messrs" for multiple male recipients and "Mses" for multiple female recipients.
Both the letterhead and the "inside address" will feature company names--that of the sender and that of the recipient. It may not be appropriate to abbreviate any part of the company name unless the company's name is registered with an abbreviation. For example, if the company is registered as "Motts, Inc.," you may use "Inc." in the inside address without spelling out "Incorporated." Likewise, do not substitute "&" for "and" unless it is part of the company name.
Unlike the front of an envelope, the state should be spelled out when it's part of your letterhead, header or inside address. For example, if the recipient's company is in Seattle, the inside address would read "Seattle, Washington 98101."
A postscript, which is an extra statement following the closing of a letter, is always expressed with the abbreviation "P.S." If you are including extra documents along with your letter, it is acceptable to indicate them either by writing out "Enclosures," followed by the number of documents, or by abbreviating "Enc." below your signature. RSVP is an acceptable abbreviation when you request the recipient respond to your letter.
The subject of a business letter may be indicated below the date with the abbreviation "Re" for "Regarding," followed by a short statement of the topic. If a letter is signed on behalf of someone else, the abbreviation "p.p." may be used. For example, if a secretary writes a letter on behalf of her employer, the typist may sign the letter and accompany her signature with "p.p." and add her employer's name underneath. The "p.p." stands for the Latin phrase "per procurationem." The abbreviation "cc" traditionally stands for "carbon copy," but may also mean "courtesy copy." In either case, it indicates multiple copies of a letter have been sent out and names the people to whom those copies have been sent.