Goal of this how-to: Step to step guide on how to setup OpenDKIM with postfix on Debian GNU/Linux to send signed email from your VPS.
There are numerous methods or techniques that you can use to achieve email message signing. Good examples are DomainKey as well as DKIM which is an abbreviation for DomainKeys Identified Mail.
DomainKeys Identified Mail (DKIM) lets an organization take responsibility for a message that is in transit. The organization is a handler of the message, either as its originator or as an intermediary. Their reputation is the basis for evaluating whether to trust the message for further handling, such as delivery. Technically DKIM provides a method for validating a domain name identity that is associated with a message through cryptographic authentication.