Internet headers are pretty handy to have access too. They can let you know the exact servers that delivered an email to you, as well as help you to figure out where SPAM is coming from.

I wanted to verify how a new mail server I'm setting up is getting it's mail out to the Internet. Since I use Gmail anyway, I always use it to test  inbound and outbound email. But how do you view the headers?

It's not all that obvious unless you know it's there.

First, open the email up. Then click the arrow next to reply, and click on "Show Original".

That's all there is to it.