It is not a just a matter of spoofing the sending IP. SMTP is based on TCP which is connection oriented. The exploit most commonly used is when one or more intermediate SMTP relays are used to relay the mail combined with dirty tricks in the SMTP protocol exchange.
I agree with PxT that there are not too many defensible scenarios to send anonymous email or mask the originating machine. The potential for abuse may far outweigh the benefit for good.