Ms. is exactly the right word for a woman when you don't know whether she's married or not. Es wurde erfunden vor etwa 30 Jahren, ist inzwischen gebräuchlich.
Mrs. is for a married woman, esp. if she has taken her husband's last name upon marriage.
Miss is for someone who is definitely unmarried, e.g. a young girl or an older lady.

I think we had this discussion earlier but don't remember the thread. Maybe anyone else reading this does?
Anyway, my English (BE) colleague recommended me to use Ms/Ms. (depending whether it's AE or BE) as this covers both and the person reading it can chose whatever is the case. Hope this helps!

Thank you very much, Nancy!

IF you know she is a female, Ms is OK,
but what do you do with

Jan Jones (Jan is male and female, and short for Janice
Pat Smith (Patrick, Patricia, Pat)
Scottie Jones (male or female?)
and there are more

Write the name only on the envelope and the opening: Dear Pat Jones,

@xyz: I can't think of any language where there's a way to place a polite and ambiguous title in front of the name when you don't know what gender the recipient is! So yes, I think your solution is the only way. I have similar problems in German and Dutch when the first name is only indicated with an initial.

@xyz: Was ist mit "Pat Jones, Esq" auf dem Umschlag? So hab ich das mal gelernt. Ist das außer Mode geraten? (Für die Anrede im Brief hilft das natürlich auch nicht weiter...)

With equality in the workplace there's no diff who's of what gender.
Forget "Esquire" and all titles unless you know the recipient insist on it.

However, never forget:

particulary in the military branch the title entirely substitute the name.

The military wish is to indicate that way, that any officer shall and do perform his duty based on fact and decision finding, never based on personell matters. Hence there is no male or female title.

You will read a text like follows when you visit the "executives bios"' pages of the U.S. armed forces: "On July 12th, 1998, Dr. Jane Miller was appointed by the president to the rank of a major general and assigned as commander of the command of the 65th Army. The general achieved a number of awards. The general got some public attention when successfully dealing with the wildfires in the generals county. The general is married with 2 children. The husband of the general is a active duty member of California's National Guard."

Start with "Dear" and close by colon.

Use the title in case you write to a warrant officer
("Dear Chief Warrant Officer:")
Use the term "Sir" in case you write to a commissioned officer
(2nd Lieutenant to Colonol)
("Dear Sir:")
Use the term "General", in case you write to a general officer of whatever rank of hirachy (Brigade General through General)
("Dear General:")
Use the term "Admiral" in case you write to a flag offier of whatever rank of hirachy. (Rear Admiral through Admiral)
("Dear Admiral:")
Close your Writing by the term "Respectfully"

I know that the military forces expect traditional conduct. If you pursue good business with military, better be sticked on thier rules of conduct.

Oh danke, werde ich bei bei meinem täglichen Briefwechsel mit der US-Army in
Zukunft beachten ... :-)

Esq. erscheint mir schon seit einigen Jahrzehnten nicht mehr wirklich in Benutzung zu sein, auch wenn ich persönlich diese Form der Anrede als die schönste empfinde

Wer halt unbedingt mal wieder "Esq." auf 'nen Briefumschlag schreiben will, kann das ja noch fuer Rechtsanwaelte tun, zumal in AE. Allerdings wird das m.E. selbst dann zunehmend belaechelt, zumindest bei juengeren Juristen.

In der New York Times wird durchweg Ms verwendet, selbst bei Frauen, von der jeder weiss, dass sie verheiratet ist (z.B. Ms Palin, Ms Bush)

Holger, wieso selbst? Ms ist neutral, geht sowohl fuer Miss (unverheiratet) als auch fuer Mrs (verheiratet).

Btw. der Faden ist 5 Jahre alt und es gibt noch tausend andere zum Thema.

Die New York Times verwendet übrigens nicht durchgängig Ms. Sie richtet sich jeweils danach, wie die jeweilige Frau es bevorzugt:
http://languagelog.ldc.upenn.edu/nll/?p=632