A computer converts elements of a mail message, neither requiring prior manual registration of conversion process details, nor increasing complexity of the system. When a mail message arrives at this computer, an analysis record registration unit produces an analysis record that gives its sender's identifier and a source mailer type and stores it in an analysis record database. Based on the identifiers found in such analysis records, destination mailer type of each received mail message is determined. A conversion database stores conversion data specifying a conversion process corresponding to each specific combination of sender and recipient mailer types. A search unit retrieves conversion data that matches with the combination of source and destination mailer types of the received message. If a match is found for that combination, a converter converts the mail message according to a conversion process specified by the retrieved conversion data.