Hi I have had an email forwarder to my blueyonder account for nearly 20 years, just recently mails are not arriving to my inbox, when sent to the forwarder email address. some still arrive some don't arrive, and some take a couple of days to arrive.
This started happening about a week ago, as though new spam software has been installed.
I have tried sending my self mails from my gmail account one sent the the blueyonder address arrived in less than a minute the one via the forwarder took days to arrive.
I think you are probably right that Virgin Media have raised the bar on their spam filtering and that has affect impacted your forwarded emails.
Are these email sent to a private domain that you have? If so, these forwarded emails 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 email server that is sending that email will be a server belonging to your hosting company.
That means that the forwarded email will fail the SPF authentication check that VM 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.
Originally VM allowed these forwarded emails through with little checking. Then several years ago they introduced checks on them but it would seem that they have recently tightened up on those checks even further. That results in the emails either getting "greylisted", which delays their arrival in your inbox, or rejected although as spoofed scam emails, which means they never arrive at all.
If these emails are linked to a private domain that you own then your first step should be to contact the hosting company and confirm that you have SPF, DMARC and DKIM records set up correctly for your domain. If you are not familiar with those terms here is a fairly simple explanation https://netcorecloud.com/tutorials/spf-dkim-dmarc/
I have had private email domains for years and in my experience the auto forwarding route is a cheap (often free) option but is fraught with problems - such as you are now finding, All email providers are tightening up on their spam / scam filtering to protect their customers and so the problem you are seeing is almost certainly only going to get worse over time.
A much better option for managing emails on your private domain is to use an off the shelf email package from your hosting company, That would give you a online account (similar to your VM webmail account that you may use to manage your Blueyonder emails) and also the facility to send and receive your emails via an email app or client on whatever devices you have. (Mobile phone, laptop, iPad etc)
You could even go the full distance and set up your own email server, but that has its own overheads. Either of those options will be a much better long term option than relying on auto forwarding,
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