Internet emails are designed to carry the IP address of the computer from which the email was sent. This IP address is stored in an email header delivered to the recipient along with the message. Email headers can be thought of like envelopes for postal mail. They contain the electronic equivalent of addressing and postmarks that reflect the routing of mail from source to destination.
Finding IP Addresses in Email Headers
Many people have never seen an email header, because modern email clients often hide the headers from view. However, headers are always delivered along with the message contents. Most email clients provide an option to enable display of these headers if desired.
Internet email headers contain several lines of text. Some lines start with the words Received: from. Following these words is an IP address, such as in the following fictitous example:
Received: from teela.mit.edu (126.96.36.199)
by mail1.aol.com with SMTP; 30 Jun 2003 02:27:02 -0000
These lines of text are automatically inserted by email servers that route the message. If only one "Received: from" line appears in the header, a person can be confident this is the actual IP address of the sender.
Understanding Multiple Received: from Lines
In some situations, however, multiple "Received: from" lines appear in an email header. This happens when the message passes through multiple email servers. Alternatively, some email spammers will insert additional fake "Received: from" lines into the headers themselves in an attempt to confuse recipents.
To identify the correct IP address when multiple "Received: from" lines are involved requires a small bit of detective work. If no faked information was inserted, the correct IP address is contained in the last "Received: from" line of the header. This is a good simple rule to follow when looking at mail from friends or family.
Understanding Faked Email Headers
If faked header information was inserted by a spammer, different rules must be applied to identify a sender's IP address. The correct IP address will be normally not be contained in the last "Received: from" line, because information faked by a sender always appears at the bottom of an email header.
To find the correct address in this case, start from the last "Received: from" line and trace the path taken by the message by traveling up through the header. The "by" (sending) location listed in each "Received" header should match with the "from" (receiving) location listed in the next "Received" header below. Disregard any entries that contain domain names or IP addresses not matching with the rest of the header chain. The last "Received: from" line containing valid information is the one that contains the sender's true address.
Note that many spammers send their emails directly rather than through Internet email servers. In these cases, all "Received: from" header lines except the first one will be faked. The first "Received: from" header line, then, will contain the sender's true IP address in this scenario.
Internet Email Services and IP Addresses.
Finally, the popular Internet-based email services differ greatly in their use of IP addresses in email headers. Use these tips to identify IP addresses in such mails.
• Google's Gmail service omits the sender IP address information from all headers. Instead, only the IP address of Gmail's mailserver is shown in Received: from. This means it is impossible to find a sender's true IP address in a received Gmail.
• Microsoft's Hotmail service provides an extended header line called "X-Originating-IP" that contains the sender's actual IP address.
• Emails from Yahoo (if untampered) contain the sender's IP address in the last Received: entry.