E-mail authentication & SSL setup

gistfile1.txt

Configuring SPF and DKIM for outgoing mail digital signing
https://www.exratione.com/2014/07/setting-up-spf-and-dkim-for-an-ubuntu-1404-mail-server/
Ref: http://www.openspf.org/SPF_Record_Syntax
Ref: http://www.dkim.org/

If Postfix not installed
https://www.digitalocean.com/community/tutorials/how-to-install-and-setup-postfix-on-ubuntu-14-04
https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-postfix-as-a-send-only-smtp-server-on-ubuntu-14-04

MX tool:
https://mxtoolbox.com/deliverability
http://www.openspf.org/Tools

Implementing SSL
[using Let's Encrypt with certbot-auto] 
https://www.digitalocean.com/community/tutorials/how-to-secure-apache-with-let-s-encrypt-on-ubuntu-14-04
[using StartSSL and manual setup] 
https://www.digitalocean.com/community/tutorials/how-to-set-up-apache-with-a-free-signed-ssl-certificate-on-a-vps

SSL checking tools
https://www.sslshopper.com/ssl-checker.html
https://www.ssllabs.com/ssltest/index.html

Ref:
Authorize senders with SPF [About SPF records]: https://support.google.com/a/answer/33786
Authenticate email with DKIM [About DKIM]: https://support.google.com/a/answer/174124
Prevent outgoing spam with DMARC [About DMARC]: https://support.google.com/a/answer/2466580?hl=en