Mapping the E-Mail Deliverability Chain

By Kirill Popov and Loren McDonald , March 15, 2006

Years ago, the email delivery chain had just a few links. You loaded your email and hit "send." After a couple handoffs, the message arrived in your recipient's inbox the way you sent it.

Today, that chain has many more links. Some block your email, others help it along. Deliverability has become a big issue for many email senders. It even spawned this column.

How concerned are you? Are you trying to improve your email program's delivery rates, or are you happy with current results? We're developing an industry-wide study on email deliverability, and we want your views. Please answer our quick survey.

Meanwhile, here's how the delivery chain has lengthened over the years, along with the trouble each new link can cause.

1. The sender. It all starts here. How you manage everything -- content creation, list management, sending protocol -- shapes your deliverability.
   
   
2. E-mail service provider (ESP) or email software. You aren't automatically penalized if you use low-grade email software or a bargain-basement ESP. But these are more likely to be associated with spammers, and you can get smirched by association.
   
   
3. Mail transfer agent (MTA). This application forwards your email to either the recipient's ISP or another MTA. Some MTAs created for consumer use weren't built to handle high volumes. They'll mismanage connections as you start pushing email through. MTAs developed for volume typically offer throttling controls and connection management to meet ISP volume thresholds.
   
   
4. Outbound ISP. Some marketers still send bulk email from personal email clients. Not only are these clients not scalable for multiple recipients, but their own ISPs could assume their computers have turned into spam-spewing zombies and block anything they send.
   
   
5. Edge networks. Companies such as Postini, Barracuda, and Brightmail sit on the edge of the receiving ISP's connection and filter incoming traffic for spam, viruses, phishing, and malicious attacks. Your messages could be delayed or filtered if they contain bad code or scripting, especially if they come in at high rates.
   
   
6. Receiving ISP. This link can stop you cold if you don't follow email best practices. It's so big, it comprises many smaller links. Corporate email servers often have a restrictive set of firewalls and filters designed to reduce unauthorized use or security risks. The smaller links:
   
   
   • ISP reputation system. Most ISPs with large numbers of consumer mailboxes have an internal reputation system that monitors your IP address for the number of user complaints and how much invalid email it's sent. If the ratio of good to bad mail is too low, your IP connection is denied. The same is true for message content, specifically URLs in the message.
     
     
   • Authentication codes. These tell inbound servers you're either authorized to send from your domain or IP addresses or you are whom you claim to be. They weed out bogus senders who hijack IP addresses or use open relays and include SPF/SenderID (AOL/MSN/Hotmail) or DomainKeys (Yahoo, Gmail).
     
     
   • Internal/external blacklists. These include in-house blacklists of addresses, domain names, and IP addresses users reported as spam, and outside blacklists, maintained by either individuals or agencies (e.g., SpamCop or Spamhaus) or collaborative efforts, such as Cloudmark/SpamNet.
     
     
   • Internal/external whitelists. An ISP's whitelists are part of its reputation system. ISPs can also consult external trusted-sender and other whitelist agencies, such as Bonded Sender, Habeas, TRUSTe, and Goodmail.
     
     
   • ISP filters. These block email with multiple sins, such as spammy content, malicious exe files, and unverified senders.
     
     
   • Third-party filters. SpamAssassin and similar programs analyze an email's content, design, coding, and sender and assign points for failed tests. They reject messages that amass too many points.
     
     
   • Challenge/response. This system-wide identity check aims to stop unsolicited email generated by automated programs like random-address generators or harvesters. The sender must respond to an automated email message before the email can be delivered. Once the sender responds, his email or IP address goes on the ISP whitelist.
     
     
   • Corporate firewalls and third-party filters. Larger companies have restrictive email policies due to security concerns. Some companies prohibit using company email for personal reasons and even block permission-marketing messages.
     
     
   • Corporate email servers. Like the ISP's servers, these use blacklists, whitelists, and filters to allow trusted senders and content while keeping suspect ones out. Companies that use Microsoft Exchange also have access to Exchange's content filter.
   
   
7. Recipients. Your email is guilty until proven innocent to recipients with installed junk-detecting devices or who delete email they either don't trust or don't feel like opening:
   
   
   • E-mail client default settings and filters. Most desktop email clients now automatically block images. Many have the preview-pane-viewing option set by default. Web-based email clients are following suit.
     
     
   • User settings and filters. Users can change default settings to allow or block images, alter the preview pane, or ban unwanted content or senders.
     
     
   • Third-party anti-spam filter programs. These scan messages for spam or malicious content after they clear the ISP server but before they hit the inbox, using either preset criteria or learned knowledge based on how the user marks the email.
     
     
   • Challenge/response. Individual users whose ISPs don't use challenge/response at the gate may use this to ban unwanted senders.
     
     
   • "Report spam" button. ISP research shows many users click the "report spam" button for any unwanted email, whether they really believe it's spam or just want to unsubscribe.
     
     
   • Bulk/spam folders. Some email that clears the ISP filter net can get routed to a bulk folder. There, it may be automatically deleted unless the user moves it to the inbox manually, marks it "not spam," or adds the sender to her personal whitelist.

Future columns will outline strategies and steps to address all these links in the email delivery chain. In the meantime, please take our brief survey on email deliverability issues. Results will be announced in a future column.

Thanks, and keep on deliverin'!

Want more email marketing information? ClickZ E-Mail Reference is an archive of all our email columns, organized by topic.

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