There are key components that contribute to good deliverability (and similarly, a good sender score):
1. List quality, regular maintenance and hygiene
Keeping your list clear of bounces and regularly removing inactive subscribers reduces the appearance of being a spammer. A clean list is comprised of regularly emailed, active subscribers who have not bounced, unsubscribed and are routinely engaging in your marketing emails. ISPs look suspiciously on email senders who have a high volume of unknown recipients, inactive recipients and regularly send to bounced email addresses. 
Bounces fall into two categories, soft bounces and hard bounces. Hard bounces are email addresses that are invalid, closed or non-existent, these are permanently invalid. Once Eloqua finds a hard bounce it automatically excludes it from future sends. Soft bounces are email addresses that are active but the email is turned away prior to delivery, this is temporary problem. Eloqua will try to resend the soft bounces in a campaign for a period of times before giving up, soft bounced emails are not (by default) marked as anything but soft bounce in Eloqua and can be emailed at a later time/date. Soft bounces are more nuanced than hard bounces, the data returned to the sender contains reason codes, which should be mined for information and processed accordingly. A full list of server reason codes can be found at http://en.wikipedia.org/wiki/List_of_HTTP_status_codes, although not all codes listed are applicable to email directly but email bounce codes can be found in the list starting with 5xx. It is highly recommended that senders build a programmatic approach to handling soft bounces. This should include logic that parses the bounce codes differently based on what the status code is relaying.
List quality also helps avoid the pitfalls of spam traps and blacklists. While no list is immune to these it is a significantly smaller risk when you maintain a fully permissioned opt-in list that is regularly purged of inactive and invalid data.
2. Spam Traps
A spam trap is an email address that was typically not ever intended for communication but purely intended to lure spam. In order to prevent legitimate email form being invited the email address is typically only published in a location hidden from view but visible to email address harvesters (which are illegal under CAN-SPAM). Since no email is solicited by the owner of this address any messages are considered to be unsolicited and therefore spam.
3. Black Lists and Filters
There are five major types of blacklists and filters.
- Third Party/Public Blacklists: Companies that publish publicly available databases of bad senders. No special credentials are required to start a blacklist but some are more popular than others and are actually used as references for ISPs and some corporate IT departments.
- Sever-side filters: These are filters such as SpamAssassin or Brightmail, they use their own sets of filtering rules to catch messages suspected of being spam. Some of these technologies use heuristics, Bayesian analysts and collaborative filtering. Many ISPs rely on customized server-side filters to filter messages. These filters may also take message volume into account and block a sender who exceeds a set transmission rate.
- Client-side filters: Recipients know that spam can make it through the various processes in place to block it and therefore can utilize tools such as Norton AntiSpam, McAfee SpamKiller or even rules built into their Outlook client to filter messages.
- Corporate blacklists: Software filters and hardware tools are available to email administrators that provide them the ability to create an internal blacklist and therefor block email originating from any organization for any reason for any length of time.
- Private/ISP blacklists: Large ISPs like Yahoo, Gmail and Hotmail are likely to maintain their own proprietary list of known spammers and problem senders. Some provide feedback loops that give senders warning they are in violation of spam rules at the ISP and some (like Gmail) provide no feedback at all.
Deliverability testing through tools provided by Eloqua or third-party services such as Litmus offer the opportunity to test emails for filtering prior to sending, thus giving a sender the opportunity to make changes before their message gets sent to the junk folder or not delivered at all. For additional details on specific filters see Appendix: Spam Filters.
4. Complaint rate
Even with a fully opted-in list and a very clean database complaints happen, recipients may hit the spam button when fatigued with your message (instead of unsubscribing) or might accidentally hit it. Regardless of the way it happens the ISP will still count this as a complaint. It is generally accepted that anything greater than 1-3 complaints per thousand (0.1%-0.3%) emails sent is enough to revoke white-listing status on an IP address. Complaint rates beyond these are likely to result in negative hits to your sender reputation and deliverability. As a rule of thumb your complaints should never exceed your unsubscribes for a campaign, if it does it should provoke research into the campaign and what might have caused the problem. Typically high complaints rates can be attributed to changes in your email programs, things like changes in send times/dates, frequency of messages, content that a subscriber feels is not relevant or using your list to promote third-party services/products or content your subscribers feel is questionable.
Blog Written By: Lauren Kincke
Lauren is an Eloqua Team Leader for The Pedowitz Group. She is an Eloqua Partner Certified Consultant, Certified Salesforce.com Administrator, and has a wide range of experience with various marketing and email automation platforms.
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