Moshitsugi (an official for conveying a message) which is also called Souja/Shosha, refers to a position in charge of reporting information to a lord, or refers to the act of reporting information itself.
In the Heian period, Moushitusgi referred to a person who was in charge of reporting information to the Emperor and Daijo Tenno (the retired Emperor). At first, Nyobo (a court lady) or Kinshin (close aide/trusted vassal) took the role of Moushitsugi, but in the Insei period (during the period of the government by the retired Emperor), favorite retainers among In no Kinshin (the retired Emperor's courtier) were working as Moushitsugi, including TAKASHINA no Yasutsune at Goshirakawa-incho (the retired Emperor Goshirakawa's Office), and Nobukiyo BOMON and Kintsune SAIONJI at Gotoba-in (the Retired Emperor Gotoba's Office); these people were involved in the government affairs through announcing Migyosho (documents for informing of decisions made by third or upper ranked authorities). Later, Moushitusgi and Souja at the Imperial Court became called denso (a position to relay messages of court's people to Emperor), and from the end of Kamakura period to the early Muromachi period, positions such as Sanmon Denso (liaison officers between the imperial court and a temple), Nanto Denso (liaison officers between the imperial court and temples in Nara) and Buke Tenso (liaison officers between the imperial court and the military government) were created and were also in charge of issuing Hosho (a document for informing lower-rank people of the decision of upper people such as an emperor or shogun). Jinja Denso (liaison officers between the imperial court and temples) as well as Buke Tenso were continued until the Edo period.
In the Sekkan-ke (the families which produced the Regent and the Chief Adviser to the Emperor), Nyobo and Keishi (household superintendent) took the role of Moushitsugi, and this custom was inherited by the Shogun families in the military governments. Aside from this, a position called Kanto moshitsugi (court-appointed liaison with the bakufu) was created in the Kamakura bakufu (Japanese feudal government headed by a shogun). Kanto Moshitsugi reported the requests from the Kamakura bakufu and negotiated with the bakufu regarding the important government affairs, together with Moshitsugi. Later, the Saionji family was, by heredity, appointed to Kanto Moshitsugi, and they gained a greater voice than the Sekkan families in the Imperial Court. In the Muromachi bakufu, the families who mediated for Seii taishogun (literally, "great general who subdues the barbarians"), were limited to several specific families called Moshitsugishu (a civil servant post in Muromachi Shogunate). Also, other than Sekkan families, Moushitsugi was placed at Monzeki (temples formerly led by founder of sect, temples in which resided a member of nobility or imperial family) and Gozan (Zen temples highly ranked by the government). In the Sekkan families, Moshitsugi was appointed to Tenjobito (a high-ranking courtier allowed into the Imperial Palace) who served as Keishi or Kaboku (a man servant), and at Gozan, it was appointed to Inryouken house-hold.
The word 'Soja' first appeared in the Heian period, but was more often seen in the Muromachi period. Moushitsugi and Soja are used in the same meaning ("Nippo jisho" [Japanese-Portuguese dictionary] defines Shoja, Moushitusgi and Toritsugite [a person in charge of informing a shogun or daimyo of the name of visitors to the residence before a meeting] as synonyms). However, in the Imperial Court, bakufu and the Sekkan families, this position was often called 'Moushitsugi,' whereas it was often called 'Soja' in Shugo daimyo (shugo, which were Japanese provincial military governors, that became daimyo, which were Japanese feudal lords) and Sengoku daimyo (Japanese territorial lord in the Sengoku period). 'Soja' was exclusively used in the Oda government, the Toyotomi government and Edo bakufu (Japanese feudal government headed by a shogun), all of which rose to the central authority from the Sengoku Daimyo. In the Edo bakufu, Fudai daimyo (a daimyo in hereditary vassal to the Tokugawa family) was mainly appointed to Sojaban (an official in charge of the ceremonies), and was simply called 'Soja,' and in charge of the duties of Moushitsugi.