flybynight

I have an E-Mail address that I reserve for giving out to Websites, but don't actually use. This attracts loads of Spam, & I download the headers & delete them unless I am after something (e.g. a password I have forgotten). I recently looked at 3 mails, which were spam. They didn't have attachments. Since then this Mail Account has been trying to mail out. The SMPT Server doesn't have a valid user name (as I never use it) so Outlook prompts me for one (this is how I know that it is trying to mail out). This account isn't supposed to mail out unless I hit the Send/Receive button. Also there is nothing in my Outbox. My 1st conclusion was that I have a worm or trojan, but my PC has passed Scans by AVG, Trend House Call, Symantec, Trojan Hunter, Ad Aware, & Spybot. Can anyone suggest what is happening? TIA