Do you mean, how did they get your email address in the first place? Or are you talking about them using your email address in the "from" field?
In the latter case, they just send all emails with the same email address in the "from" and the "to" fields. It doesn't actually come from your email account. They forge the information.
As far as getting email addresses, it's an industry in itself, and they resort to a lot of illegal activity. Even if you've been careful, the whole point of having an email address is so people can use it. One of your contacts could have had an infected computer, or they may not have used bcc when they sent a mailing to someone else whose computer was infected. Anything you've ever posted online, ever, will be harvested by bots (also illegal) and sold to companies that don't ask questions, who then sell it to progressively less disreputable companies. Once the address has been released into the wild, you'll never stop the spam. You can only use the information the spammers are dropping into your lap to make their lives difficult
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