Email Marketing News and Tips

Continued from Part I

Types of Filtering Implementations

To explain exactly how filtering systems work in detail requires an explanation beyond the scope of this article. Please see the blog article named Spam Filter Inner Workings.

Corporations and Filtering

Corporate filters are some of the most stringent filters out there next to certain governmental agencies. Corporations do not abide by any set of standards or rules when it comes to filtering. So, corporate filtering implementations vary wildly from business to business. Some businesses offer employees the use of Exchange/Server level filters, but many use Gateway level filters that exclude the email recipient from access.

Many corporations are buying into filtering services such as Postini. With Postini, the corporation has their email routed through Postini’s email servers for filtering which then gets delivered (or not) to the corporation after having been filtered. The issue with handing off corporate filtering to a third party is one of scale and confidentiality. Because services like Postini serve many clients at once, it’s possible that a certain times of the day Postini’s servers could become overloaded and lose some of a company’s email. If the company is a sales oriented business, one lost email could be enough to blow a deal. For this reason, contracting out to shared filtering services may not ultimately work for critical email accounts (even if services like Postini do eliminate unwanted emails).

Also, because the company’s emails are filtering through Postini, this possibly means that prying eyes could be viewing company confidential emails unless Postini signs a non-disclosure agreement. Even still, a non-disclosure doesn’t protect emails from accidentally becoming publicly viewable. It simply allows a company to sue Postini should that eventuality occur.

Blackholes (RBL: Real-time Black List) vs Filters

A black hole isn’t a type of filter even though some people might think it is. A black hole is a type of server level IP blocking mechanism (a kind of dynamic firewall). If an IP is in a black hole list (such as Spamcop), the IP isn’t allowed to deliver any email to the mail server subscribing to that black hole list (as long as the IP remains on the list). Mail administrators subscribe to black hole lists to reduce unwanted traffic to their mail server (just as they do with gateway filters).

Black lists are consulted early in the connection process to allow dropping the connection before any content has been delivered. A filter is different from a black hole because the filter is utilized after the email content has been accepted by the mail server, but before it’s placed into the inbox.

Black hole blocks are usually implemented as a result of excessive numbers of complaints about a specific Email Service Provider or a specific company and usually because the email content is easily recognizable as spam. Reputable Email Service Providers do not allow this type of email to be delivered through their service and, thus, should never have any IP addresses listed on RBL lists.

Grey Lists vs Black Lists

Greylisting is a process that allows mail server administrators to push off an email for later delivery. So, instead of filtering it out (with a gateway filter) or completely blocking it (using a black hole list), it tells the delivering mail server to ‘try again later’. The thinking behind a grey list is that many spamming softwares don’t ever retry sending the emails. So, when these spam softwares are told to retry later, they don’t. Legitimate Email Service Providers like Boomerang’s email marketing platform offer enhanced and extended retry mechanisms to try delivering the emails multiple times over a specified period of time.

Continued in Part III

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