Using substitute letters for umlauted characters:
Some e-mail systems cannot access the German characters.
Also you might face problem with some Windows versions/current screen fonts when trying to display the German characters. (A good example is Arabic Windows beeing unable to display German characters correctly).

If you can not make your computer access the German characters, 
follow the procedure of putting an "e" after umlauted vowels.
The following substitutes are standard in German speaking countries.

ä = ae ö = oe ü = ue ß = ss
Ä = Ae Ö = Oe Ü = Ue

German Keyboard Layout:

°
^
!
1
" ²
2
§ ³
3
$
4
%
5
&
6
/ {
7
( [
8
) ]
9
= }
0
ß \
?
`
´
Backspace
Tab
Q
@
W
E
R
T
Z
U
I
O
P
Ü
*
+
' ~
#
Caps Lock

 
A
S
D
F
-
G
H
J
-
K
L
Ö
Ä
Enter
Shift
Y
X
C
V
B
N
M
µ
;
,
:
.
_
-
Shift
Ctrl
 
Alt
-
-
Alt
   
Ctrl

You might need this layout when you don't have a German Keyboard but you have switched your Keyboard Input Local to "DE" (in MS Windows Systems)

 

Using ALT key to access German characters:

If you don't have a german keyboard, you can always type these german characters using the ALT key.
For example to type: Groß und Schön type: Gro{ALT+225} und Sch{ALT+148}n

Here is the full table:
ä = Alt+132 ö = Alt+148 ü = Alt+129 ß = Alt+225
Ä = Alt+142 Ö = Alt+153 Ü = Alt+154


You can also view an image I found on the web.