UK2 have advised that I contact Virgin Media as the UK2 mail servers are being grey listed by Virgin Media and that is causing the delays according to the email headers.
With all due respect to all concerned, I think that UK2 are being a bit simplistic there.
The fact is that forwarded email will always appear to be suspicious to any receiving email service not just Virgin Media.
The reason is that any emails sent to one of your private domains and then forwarded on to VM (or any other email service) will have the sender's email domain as either your private domain. or more likely the domain of the original sender.
However, the server that is sending that email will be a server belonging UK2.
That means that the email will fail the SPF authentication check that most email providers apply to all incoming email to block emails sent by scammers and spammers who have a spoofed (faked) the email address to disguise the real sender to make their junk / phishing emails appear to be legitimate.
If you are not familiar with how SPF works, take a look here https://www.sparkpost.com/resources/email-explained/spf-sender-policy-framework/
However to summarise, SPF is used to verify that an email has been sent from a server that is authorized to send for that email domain. Unfortunately, if you have your own domain and then arrange to forward the emails on to another email provider such as VM they will almost always fail that SPF check. That could result in the email being greylisted, flagged as spam or potentially being rejected altogether,
You say " I have proved this by sending test emails to my personal domain email addresses and to my Virgin Media email address direct with the VM email being delivered immediately and the personal domain email addresses being heavily delayed or even missing."
That is understandable.
For example, if you have sent a test email to your VM email account, the sender's address will be .......@gmail.com and it will be coming direct from a Gmail server. So it will pass the SPF check with flying colours.
Contrast this with an email sent from the same Gmail account to one of your private domains. When UK2 forward that email on to VM it will have sender's address of ......@gmail.com but will have been sent from a UK2 server. Therefore,unless (in this case) the Gmail server has been included as an authorised sender in UK2's SPFrecord it will always look decidedly suspicious to VM and any other recipient. That's because it will appear that someone is spoofing the Gmail address.
I am not familiar with UK2 but a quick online search reveals that they do have a number relatively inexpensive email hosting packages https://www.uk2.net/email-
Since the early 2000's I have run a number of private domains myself (and still do) and I soon realised that a hosted email offering was the best way to go. The other option is to set up your own email server which puts you in complete control but that has its own overheads.
Email forwarding is a cheap, often free option but it has its draw backs.
In my experience it means that you will continually be facing this same problem which ever email account you have your private domains forward to. It may work for a while but as email providers change / upgrade their security systems you will eventually face exactly the situation you are seeing here,
That's my view for what it's worth.
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