With reference to information storing a co-occurrence probability of each of plural words in association with each of distribution-destinations, the apparatus extracts, from a message to be distributed, an unknown-word that is not included in the plural words, where the co-occurrence probability indicates a probability that each word is included in a message to be distributed to each distribution-destination. When the unknown-word is present in the message at a predetermined ratio or greater, the apparatus extracts first-words included in all of a first-group of messages each including the unknown-word, calculates the co-occurrence probability of the unknown-word for each distribution-destination, based on the information regarding the distribution-destinations to which a message including the first-words and a second-word, which is included in all of a second-group of messages each including the first-words and not including the unknown-word, is to be distributed, and registers the calculated co-occurrence probability in the information.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A non-transitory, computer-readable recording medium having stored therein a program for causing a computer to execute a process comprising: with reference to co-occurrence probability information that stores a first co-occurrence probability of each of a plurality of words in association with each of a plurality of distribution destinations, extracting, from words included in a message to be distributed, an unknown word that is not included in the plurality of words, the first co-occurrence probability indicating a probability that each word is included in a message distributed to the each distribution destination associated therewith; when the unknown word is present in words included in the message to be distributed at a predetermined ratio or greater, extracting first words which are included in all of a first group of messages each including the unknown word; calculating the first co-occurrence probability of the unknown word for each distribution destination, based on the co-occurrence probability information regarding the plurality of distribution destinations to which a message including a second word, which is included in all of a second group of messages each including the first words and not including the unknown word, is to be distributed; and registering the calculated first co-occurrence probability of the unknown word for each distribution destination, in the co-occurrence probability information. 2. The non-transitory, computer-readable recording medium of claim 1, the process further comprising extracting the second word from the second group of messages, wherein the calculating includes, calculating, when the second word is included in the plurality of words, the first co-occurrence probability of the unknown word, based the first co-occurrence probability of the second word for each of the plurality of distribution destinations. 3. The non-transitory, computer-readable recording medium of claim 2, wherein the extracting the first words includes extracting a plurality of combinations of the first words, each combination of the first words having a second co-occurrence probability higher than or equal to a predetermined threshold, the second co-occurrence probability indicating a probability of co-occurrence of words of each combination being included in each of the first group of messages; the extracting the second word includes for each of the extracted plurality of combinations of the first words, extracting, as the second word, a word for which a third co-occurrence probability is higher than or equal to a predetermined threshold, the third co-occurrence probability indicating a probability of co-occurrence of the word and all the first words of the extracted each combination; and the calculating includes with reference to the co-occurrence probability information, calculating the first co-occurrence probability of the unknown word for each distribution destination, based on the first co-occurrence probability of the second word for each distribution destination, a number of the first words included in each combination of the first words, and a number of the first words included in a combination that includes a maximum number of the first words among the plurality of combinations. 4. The non-transitory, computer-readable recording medium of claim 3, the process further comprising: out of permutations of words included in each message of the first group of messages, extracting a permutation having a highest number of occurrences of a head word of the permutation being included in the first group of messages as a candidate for a combination of the first words; as a fourth co-occurrence probability indicating a probability of co-occurrence of words being included in the extracted permutation, calculating a ratio of a number of occurrences of all words included in the permutation being included in the first group of messages, to a number of occurrences of the head word of the permutation being included in the first group of messages; and the extracting the first words includes extracting, as a combination of the first words, the extracted permutation when the calculated fourth co-occurrence probability is higher than or equal to a predetermined threshold. 5. The non-transitory, computer-readable recording medium of claim 1, the process further comprising: with reference to the co-occurrence probability information, calculating, for each of the plurality of distribution destinations, a distribution probability of the message to be distributed to each distribution destination, the distribution probability indicating a probability that a distribution destination of the message to be distributed becomes the each distribution destination; and determining one of the plurality of distribution destinations having a highest value of the distribution probability to be the distribution destination of the message to be distributed. 6. An apparatus comprising: a memory; and a processor coupled to the memory and configured to: with reference to co-occurrence probability information that stores a first co-occurrence probability of each of a plurality of words in association with each of a plurality of distribution destinations, extract, from words included in a message to be distributed, an unknown word that is not included in the plurality of words, the first co-occurrence probability indicating a probability that each word is included in a message distributed to the each distribution destination associated therewith; when the unknown word is present in words included in the message to be distributed at a predetermined ratio or greater, extract first words which are included in all of a first group of messages each including the unknown word; calculate the first co-occurrence probability of the unknown word for each distribution destination, based on the co-occurrence probability information regarding the plurality of distribution destinations to which a message including a second word, which is included in all of a second group of messages each including the first words and not including the unknown word, is to be distributed; and register the calculated first co-occurrence probability of the unknown word for each distribution destination, in the co-occurrence probability information. 7. A method comprising: with reference to co-occurrence probability information that stores a first co-occurrence probability of each of a plurality of words in association with each of a plurality of distribution destinations, extracting, from words included in a message to be distributed, an unknown word that is not included in the plurality of words, the first co-occurrence probability indicating a probability that each word is included in a message distributed to the each distribution destination associated therewith; when the unknown word is present in words included in the message to be distributed at a predetermined ratio or greater, extracting first words which are included in all of a first group of messages each including the unknown word; calculating the first co-occurrence probability of the unknown word for each distribution destination, based on the co-occurrence probability information regarding the plurality of distribution destinations to which a message including a second word, which is included in all of a second group of messages each including the first words and not including the unknown word, is to be distributed; and registering the calculated first co-occurrence probability of the unknown word for each distribution destination, in the co-occurrence probability information.