When I set this site up I built a simple email server to allow to support user registration confirmation and password resets.
That was quite easy to do and it worked fine on Yahoo email and Outlook. Then I tested it on Gmail at every mail was put in the spam folder.
I googled why this happened and there were lots of possible issues but most all of them advised setting up:
I had not done this initially as it looked rather complex for my simple needs, but you really do need to set these up for any mail service.
Below are the links I followed to set these up
It is assumed that you can modify your DNS settings to add TXT records and spf records.
Apparently it can take up to 2 weeks for things to settle so I will wait before making any other changes.
I used the following link to generate a basic SPF record: spfwizard
On my DNS hosting service, I added this as an SPF record as well as adding as a TXT record
I used this guide from DigitalOcean which worked for me
I used this guide to ser up DMARC text record
After setting this up it took a little while until the test they have on the site recognised the DMARC setup.