Okay, here we go. First, let me say that modifying the mail file in any way other than adding folders is asking for trouble.
Now that I have said that, let's get down to answering the question.
Modifying the default font used is simple . . . if you have a minimum of design abilities. If not, get a developer and tell him to do the following :
1) open the user's mail file in the Designer.
2) open the form Memo.
3) scroll down to the first Body field.
4) select the field properties and change font to desired value.
6) check form properties and prohibit design/refresh of the form.
5) save the form and exit Designer.
Once that is done, creating a new Memo should use the new font you specified.
Now for the caveats :
1) This modification will block future design updates from being automatically processed by the server. In other words, your user's mail file is locked in time (well, the Memo form is, but that is almost as bad). If the server is upgraded and/or a new mail version is implemented, you'll have to track that user and upgrade his mail file manually.
2) This font change is only functional for recipients that have the same font installed on their computer (OS limitation). If the user is requiring a fancy font, stress unto him that he will most probably be the only one seeing the message in that font.
3) I do hope this user is at least a senior manager for you to go through all the trouble he's going to give you at the next round of upgrades ;-).
Pascal.