3. Destination, is the recipient of your message.
 Every email message contains a body--the actual message and a header--visible, only if the recipient chooses to view it. Message headers contain all of the information about the route that takes place when the email travels from the origin to the relay to the destination, including the origin of the country that that particular IP address “block” of that country that if belongs to. Notes: Usually entire block of IP addresses are assigned to each different county.

4. IP address, every computer connected to the Internet
is represented by an IP address and a domain name. An IP address is a numeric code (ie. 205.199.212.30) that uniquely identifies a computer on the Internet which can be traced to the owner of that particular machine being used to dial-up, ISDN, cable, DSL, ADSL, etc. to access the Internet as well as the country that it came from.

5. Domain name, is a name that corresponds
with an IP address and can be easily traced to the real owner using nslookup--a DNS whois tool that perform forward/reverse DNS queries for a domain address and getting an IP address of a hostname and hostname of the IP address.

Summary

Every email message header contains the IP address and domain of the computer from where the message originated, the IP address and domain of the computer that relay the message, and the IP address and domain of the final destination.

The information in an email header is used by many anti-spam filters to blacklist an email domain or the entire block of it, in combination with specific text, sentence string in the message, subject line, etc. See: http://www.spamhaus.org

Spamhaus, bulk or mass mailers that uses software to randomized the message *Subject header, *IP address, *Email domain, *From, *Reply To:, etc. Whatever is done to fake or forged, won't work because their entire block of IP addresses are blacklisted.

As spam filters gets more and more effective by using a cocktail of methods from; blacklisting, content or behavioral analysis--such as large numbers of blind copies (BCC), address validation nslookup--for senders trying to cloak their identity to heuristics and digital algorithms identifying common spam patterns and scanning for pornography graphics skin tones...

Spammers are resorting to: Email spoofing - Hijacking - Spam Zombies - Open relay - using fake, forged or stolen id to set-up throw-away internet access account, etc.