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This is handled by the actual transport agent.
If you are using an ISP's server to send and receive e-mails you may THINK that you are hiding your IP's but you are actually not.
The ISP's server documents the IP of the connecting machine and adds this information to the outbound e-mail's headers as it "transports" your message.
Then when the ISP's machine connects to the remote server, the remote server in turn knows the IP of the connecting ISP server.
The only way around this is to spoof the IP addresses or to send your mail via an unsuspecting third party much like spammers do.
Note however that most e-mail servers are becoming more and more aware of this stuff and most will no longer accept mail from sources with either masked or invalid IP addresses.
That is the receiving server will not accept the mail until it can verify that the sending server is who it says it is.
That said a few transport agents actually seem to delete the header entry containing the IP of the originating machine, but they must still provide their own information.
So someone can "trace" you back as far as your ISP, but at times no further in this case.
You can install your own transport in Linux (e.g. sendmail) then modify the sendmail configuration filters to explicitly exclude this stuff, and then let sendmail deliver your messages.
The problem is that your machine will be talking directly to the remove smtp server.
When your machine connects to the remote server it's IP is immediately known.
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