The sub-series consists of correspondence from MacKenzie's childhood through 1978. The correspondents in this series fall into two groups: relatives and close, personal friends. Consequently, the tone of the series differs considerably from that of the Main Correspondence sub-series. MacKenzie made this distinction himself as most of these letters were kept in bundles or boxes, separate from the Main Series. While there are a few letters from the 1890s and 1900s, the central part of this series begins after 1920. The materials are primarily handwritten letters to MacKenzie, although, for later years, there is the occasional typewritten copy of MacKenzie's reply.
The correspondence is arranged alphabetically and then chronologically. Each person's name is followed by nicknames and, where applicable, their relationship to MacKenzie.
Box 241 primarily contains MacKenzie's outgoing correspondence from World War I to family and friends. Some later outgoing correspondence, mainly to MacKenzie's family, is also included in box 241. Box 242 contains a small amount of correspondence with Marge MacKenzie.