Ensuring permission-based e-mail is delivered to recipients' inboxes requires an equal amount of effort on marketing and technical fronts. Last month's column looked at ways marketers can bolster delivery. This month, we focus more on the technology behind e-mail sending and ways to ensure your e-mail delivery doesn't resemble a spam attack.

Following, 10 tactics to help increase the likelihood your e-mail messages will be accepted by the receiving ISP and avoid future deliverability problems.

1. Create a reverse DNS. Make sure your outgoing mailing IPs have valid RDNS (define) entries set up. This ensures when a receiving e-mail server checks who owns the IP trying to connect to it, you'll come up as the result, passing one of the many basic checks ISPs do to deter spammers.
   
   
2. Set up an SPF. SPF (define) is an additional step to verify an e-mail sender's identity. The protocol is fairly easy to set up; your network administrator should be able to do it in under five minutes. SPF adds another layer of authentication to your outgoing e-mail and protects against phishing (define) attacks on your brand. Some ISPs, such as AOL, require SPF to be implemented to be considered for their white lists.
   
   
3. Make only one connection. When connecting to an e-mail server, send only one message per connection. Some systems still try to shovel as many messages through one connection as possible, akin to throwing 500 e-mail addresses into the BCC field. ISPs frown on this technique, as spammers who want to get as many messages in before being blocked typically use this approach.
   
   
4. Limit sending rate. Just because you can send a million messages per hour doesn't mean doing so is prudent. Large spikes in traffic can be seen as dictionary (define) or denial of service (define) attacks. Though the ideal send volume depends on the list's nature (e.g., B2C or B2B), a good rule of thumb is to limit your transmission to 150-200K messages per hour. Keep in mind you will also need to accept feedback in the form of bounced messages; your outgoing speed shouldn't hamper your ability to receive bounces.
   
   
5. Accept bounces. Some e-mail systems, especially older ones, have a nasty habit of rejecting bounce messages. These "bounced bounces" arrive at the receiving ISP and can raise red flags. Nothing irks an ISP more than sending a response that a recipient doesn't exist, only to have the notification rejected and the mailings continue.
   
   
6. Validate HTML content. One of the dirtiest tricks in a spammer's arsenal is invalid, broken, and malicious HTML code, used to obfuscate his payload. If you use HTML in your messages, make sure your code is error-free and follows W3C HTML guidelines.
   
   
7. Avoid scripting. Security risks due to script vulnerabilities in e-mail browsers have increased over the years. The result is most scripts, such as JavaScript and VBScript, are stripped out of messages. Some e-mail systems reject messages outright if scripting is detected. For greatest compatibility, avoid using scripts in messages. Instead, drive your readers to your Web site, where dynamic components are easily rendered.
   
   
8. Understand content filtering basics. Ignorance of filtering approaches is no excuse for not getting messages delivered. Though no one can be expected to keep up with the nuances of common content filtering, you should understand the different kinds of filters and types of content considered high risk. Read bounce messages, track which messages had high bounce rates and low open rates, and see if you can reverse-engineer offending content.
   
   
9. Monitor delivery and bounce rates by ISP/domain. Periodically (if not after sending every message) run reports by major ISP and domain on your messages. Look for unusual bounce, unsubscribe, spam complaint, and open rates at specific domains. A domain showing off-kilter results likely has a filter or blocking problem.
   
   
10. Monitor spam complaints. Even the best permission marketers with world-class practices receive spam complaints, particularly if they have a high AOL subscriber base. Monitor the number of spam complaints for each mailing, and establish a benchmark average. Look for mailings with spam complaint percentages that vary from the norm. See if you can determine what may have caused the problem. Was it an overly aggressive subject line? Too many messages sent within a short time? The fact you sent an unexpected type of e-mail? Another factor? A high percentage of spam complaints may result in an ISP blocking current, or even future, messages.