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spammers harvest mail address and sendt from address can be faked, so it may not have to been harvested from your machine/account it can be from somone you have sendt mail to

some info to read here https://www.google.cz/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=ask+leo+how+do+spammers+get+may+mail+address