I have come across a problem when sending emails from mail.app in html. Sometimes the font size on receiving machines is a different font than is set on the email client. When viewed in mail.app the fonts look consistent and correct and also appear consistent and correct when received on another machine using mail.app. However, the fonts are often of different sizes when received in Outlook on a PC or through a web based email account. The font is quite often different within the email (received) even if it is set to the same font (especially the signature).
It appears Apple's default state is not to send the email in html unless the font is altered.
How can this be set so all emails are sent in html and received correctly on the different clients?