DomainKeys Identified Mail (DKIM) defines a domain-level authentication framework for email using public-key cryptography and key server technology to permit verification of the source and contents of messages by either Mail Transport Agents (MTAs) or Mail User Agents (MUAs).
DKIM is synthesized and enhanced Yahoo!'s DomainKeys and Cisco's Identified Internet Mail specifications. It enables mail servers to cryptographically sign the outbound messages they send, and allows the recipients of these signed messages to verify that they were sent by a server which is authorized to send mail for that domain.
More information available on http://www.dkim.org
Install Debian libdkim-dev package
Replace the original qmail-remote binary by the qmail-remote DKIM wrapper script from Joerg Backschues + create spool directory
cp /downloads/scripts/qmail-remote /var/qmail/bin/qmail-remote
chown root:qmail /var/qmail/bin/qmail-remote
chmod 0755 /var/qmail/bin/qmail-remote
mkdir -p /var/domainkeys
chown qmailr:qmail /var/domainkeys
chmod 0700 /var/domainkeys
Create your key
Your keys are stored into /etc/domainkeys/your-domain/
Then you just have to put the TXT record in your DNS domain zone file !