Terminology

DKIM

DomainKeys Identified Mail: a system for digitally signing email that provides the means to securely verify that the email's origin is legitimate.

MTA

Message transfer agent: an email relay service, i.e. Postfix, QMail or Exim4.

SMTP

Simple Mail Transfer Protocol: a network protocol for sending email to a remote server, often involving password authentication and end-to-end SSL encryption.