Stamps by Countries - YEM-Kingdom
It is sometimes difficult to decide whether a stamp is an official issue and thus valid for postage, especially in African and Asian countries. If a stamp is listed in the world-wide accepted and well-known stamp catalogues Michel (MI, Germany), Scott (SC, USA), Stanley-Gibbons (SG, United Kingdom) or Yvert & Tellier (YV, France) it is a real postage stamp for sure. Often these catalogues publish new issues from exotic countries only many years after their issue date.
If it is doubtful whether a stamp is an official issue or not that particular stamp remains listed in the category "Stamps" unless its unauthorized status is proven.