I have a nasty feeling I know the answer to this one, however...
We're looking at how to enforce a corporate standard on email, obviously outbound is the priority over internal mail.
Is there any way to set the default font used in Outlook?
I believe it can be done through the registry, but GPO would be ideal.
It might not be totally what you're after, but have a look at Exclaimer. If you're on Exchange 2003 check out Mail Utilities, Exchange 2007 or Exchange 2010 check out Mail Disclaimers.
This software can give your emails a very standard look and feel, as well as pull fields from Active Directory into the email which you can use in a signature block. I don't think it can control the font of the email which has been sent - but in my experience, people don't really change the default font in Outlook.
Sorry if that sounded like a sales pitch - I'm nothing to do with Exclaimer, just a very happy customer.
A GPO can be used to set values/keys in the registry.
But consider it will be very easy for users to override the default font on each email (it might even be possible to automate this with a macro). So probably more important to educate users about what the standard is and why they should follow it.