Spam Threshold is a score assigned to a piece an email.  This score is based on a set of rules that mail is checked against for known spam issues.  The lower this threshold, the lower the score the email has to have before it is marked as “Spam”.  While the spam score can vary from 0 to 10 or more, a spam threshold of spam threshold of 6.5 is considered quite lenient, while 3.5 would be considered very strict.