Issue:
When I set the font for my outgoing new emails under Tools/Options/Mail Format/Stationery and Fonts/Personal Stationery/ and then try and send an email; the font is something other than what was set.
I have experienced the same issue and I believe I have a fix for this particular problem.
I believe this problem originates with anyone using HTML as their format for generating emails, and if they are using any Stationery Theme other than (No Theme). What happens is that Outlook pulls from a pre-generated html file that contains the theme information. This file has a pre-set font that for some reason does not get bypassed.
Example: I am using the Technical Stationery Theme. Outlook uses a file located on my C: drive as its starting place to set up the email. For me the file is located here
C:\Program Files\Common Files\Microsoft Shared\Stationery\Technical.htm
When you open this file in Word you will notice that the file’s font is probably the font that you are defaulting to instead of the one you want.
The simple fix is to 1)Find the *.htm file that corresponds to your Stationery Theme 2) Open it so you can edit it 3) change the font in the htm file and then 4) save it with the new font.
If you do this process above I believe you will have a good chance of fixing or atleast getting what you want.
To find the *.htm file I simply did a search on my C: drive for Technical.* and then I opened/edited the file in Word.
Good Luck 
Searching for on answer like ‘outlook 2007 change default font’ will give you trillions of hits, most of them questions and a number of answers (most of them not quit what you’re looking for).
The most common answer is: go to tools/options/mail format/Stationary and fonts, set the default font for new messages, and done!
The first thing you do after that you create a new mail. You got the new format, then press +n to use the default font and… the format is gone.
Then you spend the next hours to find another solution. I will now give you two options, read the rest of this message or change the search in goggle to “outlook 2007 change normalemail.dotm”, which will give you a number of thousand hits to read.
If you’re still reading here’s my findings. Outlook uses Word as an editor. The solution lies in Word, not in outlook! Word uses its own templates for blank documents, you probably heard about ‘normal.dot’ that’s the empty document template. When you create a new message in outlook it will actually start with an empty document in word and then set the font to the default that you have tried out from one of the solutions in the beginning of the document.
You need to find or create the template normalemail.dotm, open it, change the styles you prefer and save it. It should be in the same folder as ‘normal.dotm’.
Here are the steps:
1) Open Word
2) Bring up the open file dialog.
3) Choose word macro-enabled templates (.dotm)
4) Find and open the document normalemail.dotm. If you don’t find the file, create a new one by copy the normal.dotm.
5) Edit the font settings and save the document.
6) Create a new mail. Open up the menu ‘Format Text’ and looked at the styles and you will find that your changes now is available in outlook.