The copyright for any message belongs solely to the original
sender.  There is an implication that the original sender/author
will allow you to distribute copies via e-mail because they
posted it to a mailing list with the full knowledge that copies
would be made for purpose of e-mail distribution.
 
You, as a list manager, cannot say that anything posted to
your list can be distributed *as long as it is only distributed
for free*  You are granting a right which you don't have.  Only
the holder of the copyright can do that.
 
Of course, it might be argued that by sending e-mail to any
mailing list you give up the right to prevent copies of that
e-mail from being made in the e-mail medium.
 
If you want to set up a list policy whereby you clearly
state that the copyright on all material posted to the list
belongs to you as list manager, and that you then grant permission
for all messages to be freely copied as long as it is done on
a not-for-profit basis, you may be able to get away with it.  But
it might be hard to firmly establish that someone gives up their
copyright just by posting it to your list, even if you state it
clearly when they subscribe.  And who would want to subscribe to
such a list?
 
Will