The introduction of transition metals such as iron in oxides of alloying elements as, for instance, SnO$_2$ has been proven to enable higher capacities and superior charge storage performance when used as lithium-ion electrode materials. Herein, we report the evaluation of such electrode materials, precisely (carbon-coated) Sn$_{0.9}$Fe$_{0.1}$O$_{2−δ}$(−C), for sodium-ion battery applications. The comparison with SnO$_2$ as reference material reveals the beneficial impact of the presence of iron in the tin oxide lattice, enabling higher specific capacities and a greater reversibility of the de-/sodiation process – just like for lithium-ion battery applications. The overall achievable capacity, however, remains relatively low with about 300 mAh g$^{−1}$ and up to more than 400 mAh g$^{−1}$ for Sn$_{0.9}$Fe$_{0.1}$O$_{2-δ}$ and Sn$_{0.9}$Fe$_{0.1}$O$_{2−δ}$-C, respectively, compared to the theoretical specific capacity of more than 1,300 mAh g$^{−1}$ when assuming a completely reversible alloying and conversion reaction. The subsequently performed ex situ/operando XRD and ex situ TEM/EDX analysis unveils that this limited capacity results from an incomplete de-/sodiation reaction, thus, providing valuable insights towards an enhanced understanding of alternative reaction mechanisms for sodium-ion anode material candidates.