When you send an email to someone than your IP included on your email. This helps many big companies to track you down. Not all email provider do that, but many of them do. It also can track your actual geographical location.
Email provider like Gmail.com and Outlook.com does not leak your IP address when you send an email from their website or from their mobile apps. But be careful when you use third-party software like web mail client to login to your Gmail our Outlook than your IP address is leaked. That includes Mail app on your Smartphone or Microsoft outlook software on your Computer. Yahoo includes your IP address in your message that you sent that means you can be tracked down by NSA, police or anyone.
How not to leak the IP address?
The way it works is after you go to their website and click on START they will give you a random email address. Without closing that tab you have to message them an empty email on that email address. After they receive your email they will analysize if they have your IP address and display the message on the screen if your IP has been leaked or not. It usually takes some second after you have send your email.