Yahoo Mail
On Yahoo Mail you need to click "Full header" from menu "Compact header" while you see message.

click Compact header first

Now choose Full header
Headers in Yahoo Mail shown in pop up window

GMail
In Gmail you need to open 'original' email

First click for menu

In menu choose 'Show original'
GMail opens original email in new window or tab

Hotmail
As described in hotmail help:
To view the e-mail header information:
1. In the left pane, click Mail.
2. In the Folders list, click Inbox.
3. Right-click the message in the message list, and then click View source.
Please note, this does not work on the classic version of Windows Live Hotmail, you should use full version of Hotmail.


then click View source
Hotmail open new tab to show headers
