Up until now, using the confirm mode was the usual built-in preventative
option for spoofing.
http://www.lsoft.com/manuals/1.8d/owner/owner.html#2.12.6

Briefly: <pasted from a similar page>
  If you want to make your list less prone to spoofed/forged posts, you
  can use the "Confirm" parameter with the Send= keyword in your list
  header configuration(see examples below). The Confirm parameter will
  cause the list to generate a confirmation request that is sent to the
  address shown in the From line of the message being posted. The person
  at this address will then have to reply to this confirmation request
  giving the OK for the post to be accepted. This will cause an extra
  step for the posters, but this may be worth the inconvenience to make
  your list more secure.

  Examples of Send= keyword settings using the Confirm parameter:

  For private lists, use Send= Private,Confirm
  For public lists, use Send= Public,Confirm
  For Edited lists, use Send= Editor,Hold,Confirm

</endpaste>
side note:
Interesting information about SPF... thanks for that link and
information.

Janice