This educates the recipient that the email did indeed come from your server and you are willing to do something about the message if they are willing to forward you the message. Note that spammers will also see this message when they send a few test messages to see what your headers look like. This email came through the SMTP.ABC.ORG email server. This will place the information lines below the lines in the header. Rather than letting anyone relay email through your site, you may want to block relaying except for sites that have your approval.
Spammers love to spoof their reverse lookup.
However, it is very difficult to spoof IP addresses to the point that it is not worth the spammer's time. Again, don't include any blank lines. The second line shows how you can allow the host at 192.168.1.100 to relay email through your site. The third line is an example of allowing a whole subnet relay through your site. For example, this could be the subnet for your ISP that you use.