In the early days of email marketing, viral marketing used to happen by accident. Pass through messages are forwarded messages that are opened by the recipient. Typical email campaigns generate 0.2% pass-thru rate (PTR) (versus 5%-10% click-thru rate). One of our financial clients experienced a 30% PTR for an interesting link to a special calculator. But much of the novelty of email is now wearing thin. Making a message viral means making it special through a combination of offer, creative, perceived benefit and fun. Here are some general tips to maximize the viral quality of your message:

Offer an incentive. Producing a message with a quality offer or an incentive for pass-along is what viral marketing is all about.

Make it easy for your customers to forward the email to a friend.

Personalize the referral email message to show that it's coming from a recognizable source. Response rates increase dramatically when users can see that a message is coming from their friend.

Promote friendly referrals on an on-going basis. If you want your messages frequently forwarded then you should place a "forward to a friend" offer in every relevant email message that you send out.

Track and analyze the results. As with any marketing campaign, tracking and analyzing the results over time is absolutely necessary. These metrics will tell you which offers and customers drive the highest ROI.

And remember... when a customer refers a friend, the referral should not be considered an opt-in.

Kim McDonald is an Account Executive for Inbox Marketer, an email marketing and publishing company.