I have followed all the tutorials to build my own ubuntu web server from scratch and am successfully running a web server. In my mail-tester.com test, i get a 10/10. However, it looks like one single thing could be improved, and I am not sure how to fix it. Mail-tester.com gives me the following improvement message:
-0.001 SPF_HELO_NONE SPF: HELO does not publish an SPF Record
What exactly does this mean and how can I fix it so that HELO publishes an SPF Record?