DomainKeys Identified Mail (DKIM) is a method used by modern MTAs which allows an organization to sign own emails before delivery. The main purpose of DKIM is to help fight spam by associating a domain name with an mail message. DKIM uses public-key cryptography which allows the sender to electronically sign his emails in a way that can be verified by recipients. The DKIM public key is stored in DNS in order to let receivers verify both the origin and integrity of a message and the private key is used to sign each outgoing message. The DKIM signature is added as a field to the message’s header before delivery. In this article we will implement mail signing using OpenDKIM on Debian Wheezy although this configuration should run fine on any Linux distribution.