Spam email has always been a problem, and there is really no way to find out how they got your email address and no way to prevent it either. Email lists are bought, sold and traded all the time.

What I do is keep the email client Thunderbird running on my desktop. It has a really good smart spam filter that learns from the email you receive and what you mark as spam/not spam. With IMAP, even your mobile devices benefit from the filtering.

Every few days I’ll check the junk folder and mark “not spam” anything that it incorrectly flagged as spam and go through my inbox and mark spam on anything it missed. I think it miss-flags maybe 3 or 4 emails per week right now. Doesn’t happen often.