Hi,
I have posted about this issue as well.
I now have upgraded to Yosemite, and the issue still exists.
I have just analyzed the Raw Source of three emails, all of which were sent from the same source within three minutes.
Every aspect of the raw source is the same in each of the three emails.
However, one of the three prints the body of the message on the second page, leaving only the header on the first. The other two emails print as expected.
By transferring these three emails to another computer as attachments, I can also confirm that the issue still exists on that computer, therefore they are not computer or printer specific.
The space you refer to appears to be created by some sort of spacing setup that Mail is inserting. Others have already noted that printing at a smaller percentage reduces the unwanted space, therefore it is not a "page break" per se.
Has anyone actually contacted Apple regarding this?
JD