Which situation requires the use of a colon?

the closing of a personal letter

the salutation of a personal letter

the salutation of a business letter

the closing of a business letter

The salutation of a business letter typically uses a colon to set off the letter. for example ...

Dear Mr. Smith:

[letter]

or

To whom it may concern:

[letter]

personal letters are typically set off with commas (Dear Mom,). closings additionally use commas, business or personal letters (Sincerely,).