Using "Guten Tag," to adress a recipient is not very common in Germany. You should add the recipient's name:
"Guten Tag Frau XY," (female) or
"Guten Tag Herr XY", (male), respectively.
However, at least the first contact should start with
"Sehr geehrte Damen und Herren," (in case you don't know the recipient's name) or
"Sehr geehrte Frau XY," or
"Sehr geehrter Herr XY," respectively.
Edit/PS: Ahem... if this person is sort of a friend, a "Hallo Judith," or "Hallo Steffen," would be more appropriate.
[Edited 2003-06-02 12:37:22]