23. The system of claim 19 , wherein determining that the different text item received from the untrusted source that includes the phrase is spam further comprises: determining a plurality of phrases from the different text item, each of the plurality of phrases having a likelihood that the phrase is spam; and selecting, from the plurality of phrases, the longest phrases from which the different text item can be constructed; and determining that the different text item is spam based on the likelihoods that the selected longest phrases are spam.
23. The system of claim 19 , wherein determining that the different text item received from the untrusted source that includes the phrase is spam further comprises: determining a plurality of phrases from the different text item, each of the plurality of phrases having a likelihood that the phrase is spam; and selecting, from the plurality of phrases, the longest phrases from which the different text item can be constructed; and determining that the different text item is spam based on the likelihoods that the selected longest phrases are spam. 24. The system of claim 23 , wherein two or more phrases of the longest phrases from which the different text item can be constructed have the same number of words, the method further comprising: determining a respective confidence measure for each of the two or more phrases; identifying a phrase of the two or more phrases that has a largest confidence measure among the two or more phrases; and selecting the identified phrase.