The one rule under all the others
A form of address belongs to the person it names, not to the person writing. If a woman signs herself Mrs Whitfield, that is the form to use; if she signs Eleanor, the household has been offered the first name and may take it. The offer is the permission, and nothing else is.
This is why guessing at a form is so often felt as a small liberty. A guessed title announces that the writer decided something about a person before being told it. Where the form is genuinely unknown, the safe answer is the fullest form the writer can honestly support, and then to follow the reply.
On an envelope
Titles are written in full and never abbreviated on an envelope: Mister rather than Mr, Doctor rather than Dr, and no full stop after any of them. A married couple is Mr and Mrs Charles Whitfield, with the husband’s forename and not the wife’s. Where a woman has kept her own name, the couple is written on two lines, each with their own title and surname.
Initials stay inside a name where the person uses them: J. R. Hartley, not John Hartley if the person signs J. R. A widow remains Mrs Charles Whitfield unless she has said otherwise. A divorced woman who has kept the name is Mrs Whitfield, without a forename. A single woman is Miss, and an unmarried man of any age is Mr.
In a letter
The salutation follows the envelope and shortens only where the relationship allows it. Dear Mrs Whitfield opens a letter to someone known but not intimate; Dear Eleanor opens one where the first name has been offered. Dear Madam and Dear Sir stand where no name is known at all, and they are entirely correct rather than cold.
A letter to a couple is addressed to both and answered by one. A letter of condolence names the person who died; a letter of congratulation names the event. Where a title has been awarded, the title is used from the date it takes effect and not before, and a house does not congratulate a person on an honour before it has been published.
In the room
An introduction gives the older or more senior person the name of the younger first: Mrs Whitfield, may I introduce my son, Thomas. The newcomer is introduced twice, with a name and one true thing, so that two people are not left facing each other with nothing to say.
Where a person has a professional title, the title is used in the introduction and afterwards in that setting, and dropped where the room is not professional. A doctor is Doctor in a surgery, a hospital or a formal letter, and by first name at a dinner where the household uses first names. The setting decides, not the person’s preference for being addressed.
Where practice genuinely varies
Continental European practice is more formal on paper and faster to first names in person than British practice, and much of Europe uses a courtesy title with a surname on an envelope where an English letter would use a first name. In parts of Scandinavia and the Netherlands, the first name is the norm in almost every setting and a title can read as distance rather than respect.
Where a house is received abroad, the safest method is to use the fullest form on paper, watch what the host uses in speech, and follow it within the hour. A guest who follows the host’s form is doing what a guest is for.