Recent research by Return Path found that 18.7% of all e-mail sent during the second half of last year was not delivered to its intended recipient, an increase of 1.7% over the previous six months and 3.7% over the same period in 2002. The company attributed the rise to the ISPs’ more vigilant use of spam blockers, which it says are increasingly blocking legitimate e-mail.

A recent whitepaper by e-mail firm Assurance Systems offers these 10 steps legitimate e-mail marketers can take to avoid getting caught in spam filters.

1. Remove your bounces. This helps minimize the volume of e-mail you send through your ISP, increasing the odds that your legitimate messages will get through. Assurance recommends using the bounce processor on your e-mail delivery engine or making sure your service bureau does.

2. Clean your list. Most lists have typos and other errors that are readily noticeable and result in less than optimum delivery. Scan them yourself or employ a computer program to clean up the offenders.

3. Rent lists carefully. Before you rent, Assurance recommends trying out the list yourself. Subscribe and track whether you receive a confirmation of your subscription, are given the option to unsubscribe, receive too many messages and so on.

4. Make it easy to unsubscribe. Have an unsubscribe link displayed prominently on every e-mail you send as well as on your site. And make sure the unsubscribe process is easy -- it should require no more than two clicks, Assurance says.

5. Remove ‘spam-flag’ addresses. These are addresses that have been maliciously added to your list. An example Assurance uses is  abuse@somedomain.com.

6. Watch the spam keywords. Many legitimate e-mails get flagged simply for using text that tends to be used in spam. Some examples Assurance provides include: "free," the use of all caps in the subject line, excessive exclamation points or dollar signs, and excessive use of the term "click here."

7. Enable reverse DNS lookup. This provides the ISP with a way to ensure your organization is legitimate. If it can’t see your DNS, then it assumes you are a spammer.

8. Watch the server relays. Don’t relay between servers before sending e-mail out. Assurance says some e-mail marketers relay messages between servers within their control (usually in the same domain) before sending them on to the final destination. But making lots of hops between servers is one way that spammers cover their tracks, so the more hops you make, the more you look like a spammer, and the more likely it is that your messages will be blocked.

9. Monitor. You should check everything, from your delivery rates to the most popular blacklists, to ensure your messages are in the clear.

10. Be your ISP’s friend. Find out who you should contact at your ISP when there’s a problem and develop a good relationship with them.

For more information, visit Return Path here or Assurance Systems here.