Spam filtering software is designed to stop unwanted bulk advertising e-mail, known as "spam," from clogging users' in-boxes. Programs such as Spam Assassin and Postini, among others, have established screening standards for identifying and blocking spam e-mails. One feature that can trigger spam filters is a low text-to-image ratio in an e-mail message.
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In order to stop bulk e-mails and autosent messages, spam filters screen incoming e-mails for a variety of triggers, including keywords such as "free shipping" and "buy now," phrases in subject lines such as "prescriptions for less," and certain types of sender information that can identify scams, such as those originating from Nigeria. Spam filters also examine an e-mail's HTML code for specific tags and for the percentage of elements such as text and images. E-mails are assigned a spam score, and those scoring high on these red flags are blocked from appearing in a user's inbox.
E-mails and Images
Although text and images are features of many e-mails, some users choose to limit the kind and amount of images shown. Images can be blocked altogether using image-blocking controls in e-mail preferences, or blocked for specific sites only. Users who block images see only an e-mail's text, with ALT text identifying the contents of accompanying photographs and other graphics. This option affects only images embedded in an e-mail; attached files, such as photographs, are not blocked.
When screening HTML for red flags, spam filters examine the ratio of text to image. If an e-mail contains either too many images or a large single image area relative to a small amount of actual text, points are added to the spam score, increasing the chances that the message will be blocked. E-mails consisting primarily of a large single graphic with minimal accompanying text are particularly likely to score high on the spam filter, especially if other keyword flags are present.
Avoiding Spam Filters
E-mail campaigners can craft e-mails to avoid spam filters by increasing the amount of text and reducing the size and number of images in the message. For example, Lawyercasting, an online legal resource, recommends severely limiting the use of visuals in important e-mails and e-mail advertising campaigns. Although other filters may apply, newsletter formats and other types of text-based messages are less likely to be flagged for a low ratio of text to image.
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