German letter writing: The opening greeting

I am sending out some letters and need it to be as professional as possible.

If it is addressed to a man it is (for example)
"Sehr geehrter Herr Kiesel"
What if it is a Dr. Markus Keisel do I include the full title? or leave out the 'Herr'?
"Sehr geehrter Herr Dr. Markus Keisel"

What is the appropriate greeting for a female?

I also have several first names that I am not sure are male or female and need some help from those who know.

Markus = male
Andreas = male? I know of another Andreas that is a guy
Renate = ?
Sabine = ?
Dorte (with oomlaut over the 'o') =
Dagmar = ?
Axel = male? I only know of Axel Rose

Thanks for the help.
prof-

Renate = female
Sabine = female
Dorte (with oomlaut over the 'o') = female
Dagmar = female

The rest you got right. Go to the top of the class Prof.

How you doin? (with affected NY accent and emphasis on the "you")

Sehr geehrter Herr
Sehr geehrte Frau
Sehr geehrte Damen und Herren

Don't forget "Sehr geehrter Herr Prof. Dr..."

Sehr geehrter Herr Professor Doktor Dingsbums...
Sehr geehrte Frau Professor Doktor Dingsbums...

Use ALL available titles!

Don't forget though that a Professor is an additional title, i.e. not all Dr(s) are Professors. Probably obvious but stated just in case. Also, all the Andreas named people I know seem to be of the male variety.

Good Luck!

PS: "how you doin? is not bad..."Sup?" also works in a pinch.

Thanks guys.

So what about Freiherr, Nobel prize winner, Prof, Dr, and chairman of German tiddlywinks club?

Andreas is definitely a man's name(that's my boyfriend's name and about 50% of the guys that I work with are named Andreas too!)

Use titles if the guy has them; they're proud of them. But use the abbreviated forms: "Sehr geehrter Herr Dr. Kiesel".
And make sure you don't spell his name several different ways (Kiesel / Keisel)

"Sehr geehrter Prof. Whatever' or Sehr geehrte Dr. Dingsbums (for a female) is good enough. Using more than one title is ridiculous.

However, if it's a Rechtsanwalt, they become anonymous :

Sehr geehrter Herr Rechtsanwalt,
Sehr geehrte Frau Rechtsanwalt, (surely Rechstanwältin though??)

No you use all available titles up until a certain point as the gold standard.
The easiest way I find to remember is by using the full title of Porsche.
Dr. Ing. h.c. F. Porsche AG is the full company title.
If Ferdi was still alive and I wanted to write him a letter, I would have to write "Sehr geehrter Herr Dr. Ing. Porsche" to be completely correct but I'd probably leave the "Ing." out (although the gent formerly known as Herr Indoors still gets letters with that in). Remember, if it is likely that they have worked to get that title, then they will be proud of that fact. Leaving it out is very rude.
Using "Herrn" is somewhat old-fashioned now (although correct).
Some titles can be left out (e.g. Bürgermeister, Pfarrer, Präsident, Vorstand) unless you believe that the person being written to would be mortally offended by its' omission.
"Sehr geehrte Frau Abgeordnete", "Sehr geehrter Herr Ministerialrat" and "Sehr geehrter Herr Oberbürgermeister" should however be used as this is the normal form. Note that in such cases, the surname is not used.
"Professor" is an unusual one as it should be included in adresses but can be omitted in a "sehr geehrte" phrase. Why? Just look at a German Ausweis. The "Dr." is always included and forms a fixed part of the name, the "Professor" is a job title and thus can be omitted such as in the case above with "Bürgermeister" etc. If you do choose to use "Professor", well I'd usually write it in fully so "sehr geehrter Frau Professorin Müller" but "Frau Prof. Dr. Müller, Hansastr. 12, 80699 München". Note: "Professorin" unless she is Head of Women's studies of something ha ha.
In terms of Edeltitel, well then it is slightly more complicated. "Sehr geehrte Gräfin Bismarck" or "Sehr geehrter Baron Greifenklau" is the form to use, even when "von" or "ten" etc. forms a part of the name as in "Gräfin von Bismarck". "Von" is the lesser title and thus falls away in the title phrase, it is still there in the address line though.
Good luck
Katrina

Herrn is old fashioned? So in this sentence:
"Ich beglaubige hiermit...Unterschrift des mir von Person bekannten Herrn so und so" I could use Herr and still be right?

Jein. "Sehr geehrter Herrn Bloggs" is old-fashioned. "Herr" is a weak masculine noun and therefore it is correct to add the "n" and in that sentence I would certainly put it in, but because of the regular usage of "Herr" without the "n" by people who should know better it is often considered old-fashioned. It is not, however, incorrect. I've just checked it in my Hammer's Grammar to make sure though!

Katrina