You shouldn't have to add a slave dns zone, you should just at wherever your nameservers are set an A record to your linode ip then wait for propagation and set it in the rdns.
You will always see your hostname in the headers of emails you should see something like this:
Return-Path: <email@domain.com>
Received: from hostname (hostname [ipaddress])
by mx.google.com with ESMTP id q34si12325268ybk.61.2010.12.18.20.05.01;
Sat, 18 Dec 2010 20:05:02 -0800 (PST)
Received-SPF: pass (google.com: domain of
email@domain.com designates ipaddress as permitted sender) client-ip=ipaddress;
Received: by hostname (Postfix, from userid 1002)
id C219A61CD; Sun, 19 Dec 2010 04:05:01 +0000 (UTC)
To:
recipient@domain.com
From:
email@domain.com
Reply-To:
email@domain.com
Message-Id: <20101219040501.C219A61CD@hostname>
Replacing
email@domain.com with the sender, hostname with the server hostname and ipaddress with the server ipaddress
Assuming rDNS worked, give it 48 hours and it should show the new host name in the headers.
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