The Roman Catholic Church says that only marriages under its jurisdiction are sacramental.
The Roman Catholic Church has exceptions. It says that a sacramental marriage is indissoluble. It also says that Orthodox sacraments are valid. Holy Matrimony is one of the sacraments. The Orthodox are not under Roman jurisdiction. Note that this is an exception. A very clear one.
My Opinion: It seems to me that the real "acid test" of the sacramental nature of a marriage is its indissolubility. If two previously unmarried baptized people with no impediments and fully capable of consent sincerely utter vows of marriage, a sacrament of marriage takes place - inside or outside the Church sanctuary. The history of the sacrament supports this.