Say you want to find out where an email came from. Maybe it was threatening or phishing or spam. How do you gather evidence of what servers passed it to you to give to the authorities? You need the full headers. They contain all the routing information. Keep in mind, though, that it is possible the sender was making use of an open relay instead of their own mailserver.
- KMail: View -> Headers -> All Headers
- Evolution: View -> Message Source (or Ctrl+U)
- Thunderbird: View -> Headers -> All
- GMail: Hit the down arrow next to "Reply" on the message, then choose "Show Original"
Now just copy and paste this into a file and hand over to whomever is dealing with the sender.