The issue is that when you are in the email itself and not in preview the email is printed incorrectly. But in preview or just the inbox view it works fine. See the FP1 notes below.
Lotus Notes/Domino 8.5.1 Fix Pack 1 Release Notice
January 26, 2010
<b>+SPR# VNTR7XUFXC – Print Preview of an opened SMTP mail message was not displaying letterhead properly. The letterhead image was missing and the To:/From: appeared as unformatted text. This regression was introduced in 8.5. </b>