Spam Hound performs hundreds of proven tests to detect signs of spam.

The heuristic pattern detection looks for certain signs commonly found in spam.
Hundreds of these indicators are all combined along with the other filtering techniques using Spam Hound's state of the art Neural Network which learns to accurately detect spam based on your personal email usage.

An example of one such indicator is special characters used to hide the true text of the email.
e.g. "\/iãg®a" or "Va1iüm"

This has been proven a very effective technique. However spammers have learnt to avoid many of the spam indicators checked for.
As well, what may be a spam sign for someone else may not be a spam sign for you.
Never the less it is difficult to make spam look like legitimate email.