Issue is likely due to the public IP Address of the network your device is connected to appearing on one or more of Spamhaus's lists that Virgin Media use to protect their outgoing mail server. To confirm, when the issue next occurs:
Your IP addresses were removed as personal information which must never be posted on a public forum. Such information enables scammers and hackers to identify your system. Just tell us the results. Which lists did the IP address appear on,if any?
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We provide you a dynamic IP address & that cannot be changed. I have checked from my end there there is no block on your IP address from our end hence you may need to contact your email provider about this.