Русско-англо-русский юридический словарь

Study by correspondence is a form of adult education: it is normally done part-time by people who have a job to do during the day. (Guidance in the Classroom by Niels Lindhard)

People who study by correspondence normally do not have the facilities for further education near them, and so they learn "through the post". (Guidance in the Classroom by Niels Lindhard)