Some of the pertinent information that we need to collect can be obtained visually from an email client or web client (such as Gmail, Yahoo!, etc.). But some information, such as the sender's IP address and reply-to information, can only be obtained via the email header.

Messageheader: https://toolbox.googleapps.com/apps/messageheader/analyzeheader

A tool from Google that can assist us with analyzing email headers called Messageheader from the Google Admin Toolbox. 

Per the site, "Messageheader analyzes SMTP message headers, which help identify the root cause of delivery delays. You can detect misconfigured servers and mail-routing problems".

Usage: Copy and paste the entire email header and run the analysis tool. 

Message Header Analyzer: https://mha.azurewebsites.net/

Another tool is called Message Header Analyzer. 

Lastly, you can also use mailheader.org.

Note: The option on which tool to use rests ultimately on you. It is good to have multiple resources to refer to as each tool might reveal information that another tool may not reveal. 

The tools below can help you analyze information about the sender's IP address:

IPinfo.io: https://ipinfo.io/

Per the site, "With IPinfo, you can pinpoint your users’ locations, customize their experiences, prevent fraud, ensure compliance, and so much more".

URLScan.io: https://urlscan.io/

Per the site, "urlscan.io is a free service to scan and analyse websites. When a URL is submitted to urlscan.io, an automated process will browse to the URL like a regular user and record the activity that this page navigation creates. This includes the domains and IPs contacted, the resources (JavaScript, CSS, etc) requested from those domains, as well as additional information about the page itself. urlscan.io will take a screenshot of the page, record the DOM content, JavaScript global variables, cookies created by the page, and a myriad of other observations. If the site is targeting the users one of the more than 400 brands tracked by urlscan.io, it will be highlighted as potentially malicious in the scan results".

urlscan.io provides a screenshot of the URL. This screenshot is provided, so you don't have to navigate to the URL in question explicitly.

You can use other tools that provide the same functionality and more, such as URL2PNG and Wannabrowser.