so are you sayng it's not possible for them to provide DKIM authentication for my outgoing mail?
DKIM uses the domain in the email "From:" address in the email not the SMTP server so your DKIM keys are not really anything to do with VM. Even though you may be sending your emails through VM's SMTP server they cannot provide you with the key to use in your DKIM record.
The thing is that if you are sending personal domain emails they should ideally be sent via Namesco's server. As far as I'm aware, Namesco servers are smtp.hosts.co.uk and imap.hosts.co.uk.
However, I see that you have set up an spf record for your domain, so have you set that to specify that your emails are coming from Virgin Media's email servers. I assume you did, but if not the record in your DNS should be:
v=spf1 include:_smtprelay.virginmedia.com ~all
If you have the spf record for your personal domain set to specify Namesco's servers that will definitely be causing you problems. Setting it to specify that your emails will be coming from the VM servers is certainly not ideal but if you have not done that already you could try and see if you get fewer errors.
I am using (and paying loads of money for) VirginMedia's SMTP server
The thing is that VM only provide that SMTP server so you can manage your VM email accounts (so @virginmedia.com / @ntlworld.com / @blueyonder.co.uk addresses). Using your ISP's servers to send personal domain emails used to be ok in the old days ( that's what I used to do with my emails in the early 2000's) but it's definitely not recommended now. Having said that VM do not actively block you from doing it.
As far as VM are concerned their email service has only ever been just an added value service included in their broadband package and is provided at no additional cost.
Like may VM broadband customers I never have used VM's email service (or the "free" email service of any other ISP come to that) and of course many VM broadband customers only ever use the VM webmail service and so make no use of VM's IMAP / POP3 and SMTP servers.
By the way, since May 2022 VM no longer provide email accounts to new broadband customers and existing customers such as yourself can no longer create new additional secondary VM addresses. Clearly, like most of the other ISP's, the provision of an email service is not high on their business priorities going forward,
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