Probably the #1 way that spammers find email addresses is by "harvesting" them from public web pages. They have 'bots which scour the net automatically, recording any and all email addresses they can find. So if your address or your list's address appears on a public web page, it will quickly be harvested and added to various spammer's lists.
The best defense is to not post email addresses on web pages at all, but to use various forms instead to solicit input. When it comes to adding subscribers to your public mailing list, we ask that you do not post "join-" or "leave-" addresses on web sites but instead add a signup form as described here.
However, if you must post an address, you should ENCODE it so that it's not easily discovered by spammer bots. Fortunately that is pretty easy.