How can I find out what caused the spam/malware warning, for example whether it was a particular term or phrase that I used and thus need to avoid in future?
Normally Virgin Media simply just block you from sending an email if their outgoing spam filter identifies it as spam. Here is an example of that: https://community.virginmedia.com/t5/Email/Outgoing-Email-Marked-As-Spam-False-Positive but there have been many others,
When sending an email from the VM Webmail service which gets blocked by the VM outgoing spam filters it displays this error message:
"[SMTP]Cannot send message (Email body not accepted due to the following reason:
"5.7.0 SPAM Content Found (VM603) xxxxxxxxxxxxxxx....."
My understanding is that the "xxxxxxxxxxxxxxx" (a string of letters and numbers) provides information that VM can use to identify the reason it was flagged as spam.
When sending using an email app or client (such as Microsoft Outlook) you get a similar error message with an identifying string but the error code is "SMTP602".
VM always remain tight-lipped about the specific reasons for locking an email account. However (again) my understanding is it that is looking for volumes of spam being sent by malware on your network or device rather than being concerned with a single spam my email you sent.
The advice from VM in the link you posted states that you should "perform a full virus scan on all devices where you have used your email address" which does suggest that what they refer to as their "spam and abuse system" is looking at a bigger picture than picking up on a single email sent by you.
The Forum Team may be able to look into this for you but as I said earlier VM do not seem to give away the specific reasons an email account has been locked or why a specific email has been flagged as spam.
I'm a Very Insightful Person, I'm here to share knowledge, I don't work for Virgin Media. Learn more
Have I helped? Click Mark as Helpful Answer or use Kudos to say thanks