More and more large companies are implementing more restrictive email policies to fight increasing SPAM. These are mainly DMARC. Such companies include Yahoo, Cox, Verizon, and others.
As this happens, some of your legitimate email may be rejected because email servers are more restrictive.
In order to minimize the probability of your email being rejected, the following describes how to implement Sender Policy Framework (SPF) and DKIM on your already running Postfix mail server.