If you contact the major free webmail companies (Yahoo, AOL, hotmail) BEFORE lauching an email campaign, you can have all your mail get through
TIme-consuming to be sure (Hotmail especially is a bitch to get through to), but the extra sales will make up for it!
Yahoo is rather easy, all they ask is you show them the unsubscribe works, have a system in place to delete mails that bounce (We told them we use Python script with QMail, and they didnt know what that was so said 'ok cool'), etc...