Try changing the font for HTML mail in preferences to a "standard" font like
Verdana or Arial or Times. And make sure the size is also reasonably large.
Open the message and try printing again.
I've not encountered any problems printing HTML mail myself, including
airline boarding passes. If changing font in preferences does not help, you
might try this:
Assuming you are in Entourage 2008, select the Message menu, "Source". (In
2004, Source is in the View menu.) Select and copy ALL the HTML source,
which is everything starting with "<html>" and ending with "</html>".
Open Textedit; paste. Use TextEdit's Format menu to "make Plain Text". Save
the file as "ticket.html".
Now, open Safari, and use the File>Open menu to open your ticket.html file.
If it displays properly, try printing from Safari.
Good luck!