From Mail's help documentation:
Mail keeps a list of the email addresses you send messages to, and uses the list to suggest or complete addresses for you when you send email. You can delete obsolete or incorrect addresses from the list so Mail stops suggesting them, and use the list to quickly see or add people in the Contacts app.
Spam addresses are appearing in my previous recipients list. About once a month I go in and find hundreds of new addresses that I have never sent an email to and are clearly spam email addresses.
I know how to delete these addresses by hand in Mail > Window > Previous Recipients, but it is a pretty annoying process. I really want to know why they are appearing and (or) how to keep that from happening.