MikeHarvey wrote: the spam emails are appearing in my BlueYonder>Google Mail>Sent Mail folder but not in my Blueyonder>Sent folder, which is where I've told Thunderbird to stick copies of my outgoing mail.
I'm not certain but I think that suggests the mails are being sent through your VM mail account on the web, rather than from your PC. The first things I would do is:
1. Change the password on your VM Email account, (http://mail.virginmedia.com/ ).
2. Virus scan your system, particularly if it's Windows!, possibly an overkill if it's linux but it won't hurt, also run a rootkit checker.
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