Most email services implement several verification procedures on incoming email to ensure that it is legitimate, and prevent their users’ inboxes from being flooded with unwanted email (spam).
For instance, Google Gmail, among other tests, verifies that the incoming email is signed with DKIM. Servers sending email to gmail users that do not implement this signing may find that the emails sent are flagged as spam by Google Gmail servers, are delivered to the spam folder of the recipient, or are just discarded.
This post goes through the details of the implementation of the DKIM digital signature on a Postfix mail server running on a linux Debian machine. It also covers briefly SPF and other email authentication mechanisms.