Thanks to all who pointed out that the font used to dispaly a received message is, in most cases, not the font used to compose it. This is because the recipient may have chosen another font to display messages or does not have the font available.
However, the way I read the documentation, if the recipient has not selected a specific font to display messages AND has the original font installed, the message will be displayed with the font used when it was composed; if that font is not installed, the receiving end will use an "approaching" font (again, this is the way I read Apple's documentation on the subject).
Note also that I have not been able to locate the Apple Mail Preference that allows me to pick up a font to display received messages ... Apart from the font used to compose messages the only other controllable font is the one used in the display of the list of messages.