Email headers include such details as sender, recipient, subject, sending time stamp, receiving time stamps, details about the servers involved, and much more. 
They are very useful in troubleshooting common issues, such as tracking spam, correcting catchalls and bounced messages.
You can get email headers only for received messages: email headers for sent messages do not exist.

How to view email headers using Roundcube Webmail 

• Login to Roundcube Webmail.
• Double-Click to open the email message.
• Click on the Show Source Icon (The icon is a piece of paper with a S on the front). 
• How to view headers in third-party email application

How to view email headers using Apple Mail

• Open Apple Mail. 
• Double-click to open the email message.
• Choose View at top menu and select Customize Toolbars.
• Drag the Full Headers or Long Headers icon into your toolbar and save changes. 
• Now you can click on Full Headers or Long Headers to the top of the toolbar. 

How to view email headers using Outlook 2007

• Open Outlook 2007.
• Open the email message.
• Under the Message tab, in the Options group, click on the Dialog Box Launcher icon.
• In the Message Options dialog box, the headers appear in the Internet headers box.

How to view email headers using Outlook 2000, 2002 and 2003

• Open Outlook.
• Double-click to open the email message in a new window.
• Select View > Options from the message's menu.  
• All header lines appear under Internet Headers at the bottom of the dialog that comes up. 

How to view email headers using Outlook Express

• Open Outlook Express
• Right click on the message and select Properties
• Choose the Details tab and select the Message Source Button

How to view email headers using Mozilla Thunderbird

• Open Thunderbird
•  Double-click to open the email message
• From the menu bar select View > Headers then choose All
• The headers should be immediately displayed
• How to view headers in third-party webmail clients

How to view email headers using Gmail

• Login to Gmail 
• Open the message you'd like to view headers for.
• Click the down arrow next to Reply, at the top-right of the message pane.
• Select Show original.
• The full headers will appear in a new window.

How to view email headers using Yahoo

• Login to Yahoo webmail
• Open the message you'd like to view headers for.
• Click Full Headers at the top of your message.
• The full headers will appear above the message text.
• How to view email headers using MSN Hotmail

Login to MSN Hotmail

• Select Options from the top MSN Hotmail navigation bar
• Make sure the Mail category is selected
• Choose Mail Display Settings
• Set Message Headers to Full.
• Click OK.
• Now you can go back to the MSN Hotmail Inbox (or any folder) to open a message with full headers.

How to view email headers using Windows Live Hotmail

• Login to  Windows Live Hotmail
• Click on the desired message in the list with the right mouse button.
• Select View source from the menu.