While it is true that Outlook always uses the format of the original message
- this just means plain text, RTF or HTML. Your own setting will determine
the font for the body of the message. Finally, after much playing around I
have discovered the trick to ensuring your font choices are maintained
separately for your signatures.
Look for your signature folder:
C:\Documents and Settings\<username>\Application Data\Microsoft\Signatures
(you may also have one under ...\Microsoft\Outlook but not sure when or if
this is still used - doesn't seem to be under my installation - Office 2003).
Make sure you have a copy of each signature in each possible format:
signature.rtf
signature.html
signature.txt
Emails in a format other than the format for which you have saved your
signature will default to the fonts/colors of the email (or so it would seem
from the behaviour).
Hope this helps!