A method performed by a receiver system, comprising: receiving a certificate that identifies a sender system as owning a domain; sending an electronic message to the domain identified in the certificate, the electronic message sent via a mail exchange network, wherein the mail exchange network correlates the domain to an address that a true owner of the domain has registered with a mail exchange authority; receiving a response from the address that the mail exchange network correlates to the domain, the response indicating that the certificate is associated with the true owner of the domain; and authenticating the sender system as the true owner of the domain based on the response; wherein: the certificate comprises a self-signed certificate signed by the sender system and the authenticating is performed independently of a certificate authority; the certificate includes a public key; the electronic message to the domain identified in the certificate contains a secret message encrypted using the public key; the response from the address that the mail exchange network correlates to the domain contains the secret message as decrypted by the domain using a private key associated with the public key and authenticating the sender system as the true owner of the domain comprises verifying the decrypted secret message. receiving a certificate that identifies a sender system as owning a domain; sending an electronic message to the domain identified in the certificate, the electronic message sent via a mail exchange network, wherein the mail exchange network correlates the domain to an address that a true owner of the domain has registered with a mail exchange authority; receiving a response from the address that the mail exchange network correlates to the domain, the response indicating that the certificate is associated with the true owner of the domain; and authenticating the sender system as the true owner of the domain based on the response; wherein: the certificate comprises a self-signed certificate signed by the sender system and the authenticating is performed independently of a certificate authority; the certificate includes a public key; the electronic message to the domain identified in the certificate contains a secret message encrypted using the public key; the response from the address that the mail exchange network correlates to the domain contains the secret message as decrypted by the domain using a private key associated with the public key and authenticating the sender system as the true owner of the domain comprises verifying the decrypted secret message. the certificate comprises a self-signed certificate signed by the sender system and the authenticating is performed independently of a certificate authority; the certificate includes a public key; the electronic message to the domain identified in the certificate contains a secret message encrypted using the public key; the response from the address that the mail exchange network correlates to the domain contains the secret message as decrypted by the domain using a private key associated with the public key and authenticating the sender system as the true owner of the domain comprises verifying the decrypted secret message.