My wife's grandmother (Annie Pieniążek, age 9) emigrated from Poland to Canada in the early 1920s. She travelled from Liverpool to Halifax on the "SS Canada", leaving 10-Feb-1921, and arriving 19-Feb. From there she went on to Winnipeg, where she lived the rest of her life.
I've found the Canadian arrival records, and the Liverpool departure records. The Liverpool departure records say that they (her mother, brother, Annie, and sister) came from Poland (natch) and arrived in Dover on the "Belgian Mail Packet"
How can I find records about that service? I've contacted the Belgian National Archives (arch.arch.be) and they said: "The National Archives of Belgium conserves no passengers lists."