Set Up Postfix For Relaying Emails Through Another Sendmail server

To configure relaying on your Postfix mailserver, you need a valid email account (with username and password) on another mailserver (provided that this mailserver makes use of SMTP-AUTH (which it should do).

Install DKIM and SPF in cPanel

This how to describe how to add domainkeys and SPF (Sender Policy Framework) records on CPanel Servers. Whenever you create a domain on CPanel server using WHM, it wont add domainkeys and SPF records for particular domains. To add domainkeys and SPF records please use following steps.
Please note on CPanel servers most of the work is done using cpanel username which then reflect setting on particular domains. If you have several domains / sub domains under one username, it will add domainkeys and SPF records for all domains / subdomains.