Recently incoming emails to my btinternet account started having the statement "[SPAM]" inserted in the subject line.
I am using Windows Live with AVG virus protection.
Also in Windows Live I have a gmail account which is not affected in the same way.
When accessing btinternet via the web, or using Yahoo mobile phone app, messages are not affected.
The affected messages are from contacts in my address list, often from private senders sending only to my address.
The only ones currently not being affected are from Amazon.