Twenty percent of email in the United States and Canada is still not making it to the inbox while 3% of email goes to the "junk" or "bulk" folder and another 16% goes missing, according to Return Path's Global Deliverability Benchmark study for the second half of 2009.

The stats are slightly worse for the B2B sector, which has a delivery rate of just 75%. These are not pretty numbers for email marketers, Return Path concludes. "Deliverability is still a crisis for commercial senders."

There are a number of reasons why these numbers are low.

1. Many senders still don't have access to reliable data on their deliverability situation. According to Return Path these senders are instead relying on reports that show a "delivered" metric that tends to be 95% or higher.

"But in most cases this is not the Inbox Placement Rate - the percentage of mail that actually arrives in the inbox. Rather it is simply a reporting of the number of messages sent minus the number that returned a hard-bounce message. This creates a false impression that nearly 100% of email messages arrive as intended."

2. Senders are leaving a lot of money on table - on average about 20%, Return Path also found. This is because marketers find it easier to concentrate on the money they can make - especially since email is a low cost channel. But such thinking make it easy to disregard or overlook lower deliverability rates, Return Path argues.

"For example, if you deploy a campaign to one million subscribers with an average conversion rate of 5% (50,000 buyers) for a value per conversion of $10, you should earn $500,000 in revenue from that campaign. But if 20% of your email never made it to the inbox, then the numbers look very different. With an 80% inbox placement rate, only 800,000 subscribers would receive your email and at the same conversion rate and value, your email revenue drops down to $400,000 for that same email campaign."

Multiply that by the number of campaigns sent every year and it turns into millions in lost revenue. "What senders fail to realize is that those dollars can be recouped simply by maintaining a good reputation and implementing basic best practices that are proven to improve inbox placement. While achieving 100% deliverability is key, even a 5% increase to your inbox placement, results (for this example) in $100,000 more in revenue per campaign."

3. Senders are still failing to implement best practices around reputation and behavior. Reputation is the driving factor that determines whether or not email makes it to the inbox, Return Path notes. However not all use such tactics as welcome messages or easy opt-out procedures.

4. Senders' have poor policies on handling bounces. The amount of email disappearing doesn’t align with the philosophy of the ISPs, writes Word to the Wise.

Yahoo and increasingly other ISPS are using temporary failures as a way to regulate and limit email sent by senders with poor reputations, issuing 4xx 'come back later' messages. When an ISP issues a 4xx message during the SMTP transaction the email should be queued and retried, Word to the Wise says.

"Modern bulk MTAs (MessageSystems, Port25, Strongmail) allow senders to fine tune bounce handling, and designate how many times an email is retried, even allowing no retries on a temporary failure. What if the missing mail is a result of senders aggressively handling 4xx messages? Some of the companies I’ve consulted for delete email addresses from mailing lists after 2 or 3 4xx responses. Other companies only retry for 12 – 24 hours and then the email is treated as hard bounced."

5. Global campaigns require different approaches. The Return Path survey also found better deliverability rates overseas. In Europe, 85% of email arrives as expected with 3.6% ending up in the "junk" or "bulk" folder and 11% not delivered at all. Asia Pacific does better than Europe and North America with 86.9% of email delivered to the inbox, 3% is "bulked" and another 10% is missing.

The difference between the US and Europe is due to ISPs' approach to filtering, said George Bilbrey, president of Return Path. (via DM News). “North American ISPs tend to use reputation more when deciding if an e-mail gets delivered, and European ISPs do much more content delivery filtering,” he said.

In China, e-mails were more likely to get delivered if they were sent from within the country. "This is not commonly the case," said Bilbrey. "We recommend that senders who target Chinese consumers work with an e-mail provider that sends from within China."