參考資料：
Setting Up SPF and DKIM for an Ubuntu 14.04 Mail Server
How To Install and Configure DKIM with Postfix on Debian Wheezy
https://www.digitalocean.com/community/tutorials/how-to-install-and-conf...
https://www.linode.com/docs/networking/dns/introduction-to-dns-records
http://serverfault.com/questions/52830/dkim-sign-outgoing-mail-from-any-...
測試工具
- http://www.mail-tester.com/
- http://www.protodave.com/tools/dkim-key-checker/
- 發送一封空的email到 check-auth@verifier.port25.com 會回覆一封檢查結果
- 檢查垃圾郵件指數(分數越低越好) https://spamscorechecker.com/