User IDs in addresses to which an e-mail is addressed are logically ordered into a 2D matrix of rows each having one user ID. For each column in the 2D matrix, a column count is found by counting the highest number of matching characters in the column. A column quotient is calculated for each column by dividing the column count by the total number of rows in the 2D matrix. A weighted column quotient is calculated for each column by multiplying the column quotient by a weight that is based on the left-to-right order of the column in the 2D matrix. A numerical assessment is made of the similarity of the user IDs in the addresses to which the e-mail is addressed by adding up the weighted column quotients for the columns in the 2D matrix.