I'm not sure how much I'm allowed to say at this point, however it looks like you are being blocked by certain anti spam provisions (ones I strongly disagree with BTW) used to protect their outbound mail relays.
I understand their intent, but good intent is not enough to justify this one.
However one thing I would suggest is that you run a malware scan on your PC - using the free version of Malwarebytes
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.
Hi, My wife and I are having the same problem sending emails from 5 different devices. I get the same error code - SMTP (smtp.virginmedia.com): <rx> 525 5.7.13 Authentication Denied (VM305) - when I run the log but with Thunderbird. Is there a fix for this or do we now need to use the web for sending all emails
Anti viral software does not exist for iOS due to it being impossible for it to actually do the job it (used to before it was banned by Apple) claimed to do. Sandboxing prevents apps from accessing files belonging to other apps or the OS. This also means you shouldn't be able to get a virus on iOS due to the sandboxing and walled garden environment.