Leaving an address in online posting such as Usenet is one way that spammers find email addresses. Spammers have programs built to scan for addresses within the text as well as the headers of the item. Mailing lists are also a very common way for people to access addresses of others. Spammers use these lists in a few different ways. They first ask the mailing list to give them a copy of the directory of addresses. If this does not work, they then try to get past the systems anti-spam configuration by sending email with subject lines that appear to be legitimate writings to the list. They make some of the mail transfer agents think that they are sending a confirmation email back to the sender, rather than the junk mail.
Other methods include, finding addresses from web pages. If a user has their email anywhere on their web page, or someone else’s, a spammer can easily gain access to it. Anywhere an email address is publicly listed is in danger of getting spam sent to it. For example, when users set up profiles online when they register to use a product such as AOL instant messenger or IRC (Inter Relay Chat). Another type of web page they will check is the online yellow and white pages. These have an abundant amount of addresses of people from all over the world. Many users are not even aware that they have been included on these pages. They do not know about it because someone else such as their email providers has signed them in. All of these methods and more can be found at "FAQ: How do spammers get people's email addresses?"*