DomainKeys Identified Mail (DKIM) is another email validation system designed to detect email spoofing. Because the MTA automatically signs email parts with a private key, the recipient can check the validity against the public key stored in the sender's DNS. Most modern mail servers should be equipped with a system that generates and verifies these signatures, and poste.io is no different.
For more information about DKIM, visit http://www.dkim.org/.
Have you found a bug in the service? Please send us the link to the broken report and we will fix it as soon as possible.