Sir and Madam (and ma'am and dame) when starting a letter or email and when it comes before a name as an honorific. Lowercase "sir" and "madam" in other cases.
When you begin a letter or an email, you must capitalize. If you're using "sir" as an honorific before someone's name, you should also capitalize it. In all other cases, sir should be written in lower case.
A second-person title is capitalized. Respectfully, Your Excellency But no salutations, Mr. Chairman. Your Excellency, Madam, Chairman, and General Mr. Your Honor, I move that the Senate approve a resolution honoring the late Senator John McCain.
A motion to proceed to a bill or resolution requires only a simple majority of those present and voting. Since this is a second-person title, it is required to be capitalized.
In American English, the title of a person who is not also used as a proper name is usually a surname preceded by the formal designation "Mr." or "Mrs." A parent's first name can be used in place of their surname if the two names are used separately and they would not be considered joint surnames. For example, Charles Manson's father's first name was Charles but he was never called Charlie because that was his father's nickname.
In British English, the usual practice is to use the surname without any prefix or honorific. So John McCain would be referred to as just "John McCain".
However, there is some variation between regions as to how much formality is expected from people who do not share the same last name as the one before it.
Yes, capitalizing Sir makes the sentence more professional. Both, according to Ashwin Joshi, are right. Most of the time, though, we write "Dear Sir," which is a more formal salutation.
Pet names or endearments, such as sweetheart, honey, dear, darling, son, bro, boy, dude, gentlemen, ladies, and so on, should also be in lowercase, even when used in direct address.
I believe the guideline is to capitalize the first and all essential words in a letter's greeting. Dear Mom and Dad, Dear Parents, Dear Students, Dear Members, Dear Teachers, Dear Production Manager, Dear Employees, and so on. A letter to a huge group should also be addressed as "Dear All." There are exceptions, though; someone writing a formal letter to a small group of people might write: "Dear Mr. Johnson and Ms. King" or "Dear Members of the Board." Capitalization is also used for foreign governments.
Essential words are those that give information about the letter's content without needing further clarification from the reader. If you're writing to tell someone that they've been awarded some prize or honor, then you would include their name in your letter. You could also include the name of someone important to the recipient, such as their employer or professor. However, if you were writing to tell someone that they had been fired, it wouldn't be necessary to include their name. They already know who it is that you're writing to them about.
Students are considered essential words because they give information about the recipient of the letter.
When used as part of a name or to address someone directly, do not uppercase titles such as president, prime minister, and governor. Otherwise, in formal writing, lowercase them. In certain styles, such titles are capitalized when referring to a specific individual. For example, "The president has been awarded the Nobel Peace Prize."
Titles can be difficult to write because there are no standard rules on how they should be formatted. Each style guide will have its own instructions on how to format titles. The best advice is to follow the conventions set out by those styles.
In English language newspapers, the title of a book, movie, or other work is usually given in caps. This is true even if the work in question does not have a cap-sensitive alphabet (such as Japanese characters).
Governors' names also tend to be written with all capitals, except for the first letter of their state name. For example, "George W. Bush" but not "George Washington Bush". President's names are always written with a capital P, except when they are quoted within a sentence (e.g., "The president said...").
Titles should be written in full without abbreviations, acronyms, or symbols. An exception would be titles that are commonly used as abbreviations, such as Dr. for Doctor or Mr. for Mister.
You must begin with Dear. Colleagues should not have their names capitalized. It is a sign of respect to use Mr. or Ms. instead.
Capitalization is used to distinguish words that are part of an official title from those that are not. For example, the name of this journal is Capitalize. Journal of Comparative Literature, where my work is published, does not have a title page that includes its name; rather, it is called a "confidential" publication. The editors and other members of our staff are referred to as "editor" or "authors," not "Mr." or "Mrs." Although most journals do not require a formal letter of acceptance, many authors prefer to receive one form letter of rejection followed by several letters of acceptance. These letters are usually sent to the different editors who made contributions to the volume. They are also sent to the author's name(s) listed on the cover of the journal.