Training a spam filter can take a good deal of time and effort. One of the best ways to jumpstart the spam filter process is to have a whitelist of email recipients. There is no better source of non-spam email addresses than the recipients of your own send folder.
Using Python's mailbox module, you can cull out the recipients of all your past email. This guide was inspired by a script by Noah Spurrier and was adapted from a similar script in O'Reilly's Python Cookbook (prices).