That x.x.x.x would be the originating IP.. however, mail relays, proxies etc will complicate this.. thus you look for the X-Originating IP header.
Well, this post reminds me of the fact "sahirh" that though i know how to decipher a little bit of those headers but there are still some places i falter on. Is there any specific resource where i can find what each & every entry in the header means ?
Next would be SP (Service Provider)
CCNA, CCNP (Switching), CCIE#20640