So does anyone know why Virgin blocking them? And is there anything that can be done?
I am not a member of VM staff so I cannot give a definitive answer but I have an good idea what the issue might be.
I have just checked and your domain "park-church.uk" does not have a DMARC or DKIM record set up. Virgin Media make use of those records to check each incoming email to ensure that the sending address has not been "spoofed" (faked).
So, because they cannot check the authenticity of park-church.uk emails, they probably flag the incoming emails from your domain as being spam "just to play safe" and protect their customers.
"I've tried flagging those messages as "not spam" countless dozens of times, maybe hundreds. It's made no difference at all."
That's right, VM actually say that flagging an email as "not spam" simply moves it to the inbox, it does not stop future emails from being flagged as spam.
I suspect the reason your emails do not get flagged by other email services may well be down to the fact that other email services have better methods of allowing customers to whitelist incoming emails that would otherwise be flagged as spam.
Here's an overview of DMARC DKIM and SPF (your domain already has an SPF record) SPF DMARC & DKIM Explained
However, your best bet is to contact the Domain Registrar that was used to obtain the park-church.uk domain, so perhaps GoDaddy, IONOS or whoever. Alternatively, that company may well have step by step instructions on their online support pages of how to set these records up.
I hope that makes sense,
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