A couple of days ago, people started getting spam emails alleging to be from my primary email address. I access the internet via BT, (which uses BT Yahoo for email) and I use Thunderbird from my PC, but more often access my email via BT webmail from my netbook.
I spotted it before people started telling me, as I got two to my work email, which I occasionally send documents to from home, and also to my one of my two secondary email addresses, which I had occasionally emailed from the primary one as a test. The emails I got had been addressed to three or four other email addresses alphabetically adjacent to mine, and included people I haven't emailed for many years and in one case, an incorrect email address that I had attempted to email but had bounced back. Some of the addresses were people I hadn't emailed for 6 or 7 years, and had cleared out my 'Sent' and 'Deleted' boxes since then, but Thunderbird must have the emails saved somewhere, because when I tried searching for one of them in my sent folder, several emails I had sent to him popped up even thought they weren't apparently deleted - this is disturbing, does it mean it someone has got into some stored Thunderbird folder on my PC?
The emails were sent out at a time when I probably had one or the other switched on, but I can't remember which; I have Malwarebytes and Superantispyware, which I've updated and run complete scans on both computers, and also Avira, and all three found nothing. The spam messages weren't in the 'sent' folders of either Thunderbird or my webmail; if I copy and paste the email address, it is mine, but I clicked on 'viewed source' and the isp they were sent from obviously wasn't mine. I copied all that information into a word document and saved it.
I've changed the password on my primary account but I don't know what to do next; there don't appear to have been any further emails after the original ones (so far) but I don't know how it happened, and I'm not convinced that there isn't something lurking on my computer.
Please can you help?