Dear Mr Brown
Yours sincerely
Envelope: Mr John Brown or John Brown, Esq

Esquire (Esq) should never be used in conjunction with any other title, as for instance the prefix Mr.

Dear Mrs Brown
Yours sincerely
Envelope: Mrs John Brown or Mrs Jane Brown

The traditional use of the husband’s first name in place of the woman’s actual given name can be political and preference varies from person to person. Check before using, if in doubt.

Widows traditionally retain their styling as Mrs John Brown until they remarry or choose otherwise.

Dear Miss Brown
Yours sincerely
Envelope: Miss Sarah Brown

Dear Ms Brown
Yours sincerely
Envelope: Ms Sarah Brown

In business, unless you know if the woman is married or not, always use Ms unless advised otherwise.

If a person’s gender is unknown or unspecified it is best to make every effort to find out so the correct honorific (Mr, Miss etc) can be used.

If a person’s gender is deliberately unspecified (e.g transgender or gender-fluid) then use the honorific Mx.

Dear Mx Brown
Yours sincerely
Envelope: Mx Alex Brown

Even though they are abbreviations in British and Commonwealth usage there is no full stop/point/period after honorifics such as Mr, Mrs, Ms, Mx, Mstr, Dr or Prof.

In American Usage there is, however.