Automatic email suffix problem - SMTP

I have an odd problem - bear with me here:

When you send an email to SMTP is confused by the space between John and Doe and as a result automatically suffixes the the email with it's own local domain (@internal.com) and sends the email as "john@internal.com"

Is there a way (outside of educating my users!) to configure Exchange to NOT automatically append a default domain on emails and instead simply have the flawed email address bounce back to the sender?