Most likely, what happened was, your e-mail address was guessed. There are programs out there that do nothing but send e-mail all day, every day. They randomly (or not so randomly) send out e-mail to every possible server they can find. I had a problem a few years back when someone hacked into my server's database and found out all of the e-mail addresses that were associated with my websites (I had no less than 35 e-mail addresses at the time). Every one of them started getting SPAM on a regular basis. It took years to stop. Some of those addresses were NEVER used for anything. Not even test e-mails were sent through the accounts.
There are things you can do.
First of all, check the headers of the incoming e-mail to find out where they are being sent from. The "From" address is not always the originating address. You can send copies of the offending e-mails to your ISP and his ISP, including the header information in your e-mail. In most cases, you can at least get him to stop. Some e-mail providers are much more aggressive with their response. I was getting SPAMMED by a guy from a Yahoo e-mail address over some motorcycle parts I posted on Craigslist. I reported him to Yahoo. Within a day, the SPAM stopped and as a test, I sent an e-mail to him, from another account. It came back undeliverable. The account was closed.
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