Support for 8-bit data exists via the Extended SMTP protocol (ESMTP). For you to succeed in getting an 8-bit message to your destination, you'd need a guaranteed 8-bit path to your destination. You can verify support for ESMTP on any mailsystem by telnetting to the sendmail port ("telnet host.name.com 25") and getting the EHLO greeting message (contrast with HELO for a non-extended mail site.)
The issue is further complicated by the problem of 7-bit vs. 8-bit for any arbitrary TCP connection. Even if a computer could speak directly to outside hosts, the connection to the target host might not be cleanly 8 bits. Internet gurus lament that we will never shake the 7-bit heritage of the Internet.
Thanks to Allen Cypher and Erik Fair from Apple Computer for this information.