IP (Internet Protocol) addresses allow computers to uniquely identify each other so that they can deliver data over networks to the appropriate computer. IP addresses also apply to email. Both the sending and receiving IP addresses are logged in the actual email, but you may have never seen this information because most email programs hide the email's background information in "headers" that they don't typically show you. To locate a sender's IP address, you have to enable these headers.
View the email's header. This procedure varies between programs. Some programs allow you to right-clicking the email and selecting "Properties." You may also have to select the email, click "File" and select "Properties" from there. Web-based emails generally have an option for viewing headers located in a drop-down menu at the top of the email screen. Select "Show Full Headers," "Show Original" or "Show Message Source" from this menu. As an example, in Gmail, you would click the small triangle to the top right of the email message and select "Show Original."
Look through the header text and locate the text that reads "Received: from."
Read across and look for an IP address in brackets. It should look something like "[000.000.00.00]." This is the sender's IP address. If there are multiple "Received: from" lines that contain IP address, the last listing will be the sender's IP address.