Encourage free members to upgrade to a paid subscription. Now, when you deliver a paid-members-only post to a someone without a subscription, they'll be able to read a free preview right in their inbox. At the end of the preview, free members will be invited to subscribe to continue reading.
All you need to do is insert the public preview divider line in your post to control where the free preview cut-off should be, and you're all set.
Paid members will see the full post content, while free members will only see the content above the public preview line — both in email and on the web.