Is solipsys.co.uk your domain (from an earlier comment)? If so, I can see right away that there's no DMARC entry for it. That means that it's vulnerable to spoofing (I or any attacker could easily send an email to someone making it look like it came from your domain, eg, as part of a phishing attack). Gmail definitely penalizes domains without DMARC, making emails more likely to be classified as spam.
I am, for many reasons, so thank you for understanding that.
> If you want to keep believing that your ISP knows better than them, then I guess you're going to.
It's not that I believe my ISP knows better than Google, it's that I believe they are doing what they have been told is necessary.
> ... solipsys.co.uk ... no DMARC entry for it.
I will raise a ticket and get it dealt with, and as a part of that I can get the entire issue looked at more closely.
Thank you.
> ... now you need to understand a lot about DNS and those mail related standards and do a significant amount of work to stay on top of the configuration to have a snowball's chance in hell of your mail getting through.
That's exactly what I pay someone else to do, and they are doing it correctly (as best I can tell).
So ... I just have to live with it. As far as I can tell, short of simply giving up entirely and using Google, I've done everything I can.