How to find out the IP address of email sender in Gmail

There is no technical way to get the ip-address of someone sending an email via the gmail web interface. Google does not put it into the email headers. And there is no API to query gmail for it.

If you really need that IP address for valid reasons, you need to get a court order.

This can potentially be done through some sort of social engineering, if you can send an email back to the original sender and get them to click on a link in the email.

You need to set up a website which logs the visitor's IP address, then somehow make the person who sent you that email visit your site. This probably involves sending them an email and hoping they read the email and click a link in it.

A related approach is to send them a HTML email that contains an inline image hosted on your website. However, most email clients do not load linked images by default, precisely to protect the privacy of their users. Therefore, in most cases this will not work (unless the person just happens to have overridden the defaults in their email client).

Therefore, a more realistic approach is to send them an email containing a link, and hope they will click on the link.

Example PHP code to log the visitor's IP address (from http://www.binrev.com/forums/index.php/topic/20288-php-ip-logger/):

<?php
header('Content-type: image/jpeg');
readfile('image.jpg');
$filename = "logged.txt";
   $somecontent = "Connection from:".$_SERVER['REMOTE_ADDR']." at ".date("r",time())."\n";
   if ($handle = fopen($filename, 'a')) {
           fwrite($handle, $somecontent)
           fclose($handle);
   }
?>

GMail does send the IP-address in its mail headers: check the X-Originating-IP header.