Sender Policy Framework
http://ja.wikipedia.org/wiki/Sender_Policy_Framework
自宅メールサーバから、Gmailにメールを送ると、
Received-SPF: neutral (google.com: 198.51.100.123 is neither permitted nor denied by best guess record for domain of hoge@mail.hoge.com) client-ip=198.51.100.123; Authentication-Results: mx.google.com; spf=neutral (google.com: 198.51.100.123 is neither permitted nor denied by best guess record for domain of hoge@mail.hoge.com) smtp.mail=hoge@mail.hoge.com
Received-SPF: neutral
Authentication-Results: mx.google.com; spf=neutral
となるのでSPFに対応させてみようと思います。