Rickart’s theorem states that every bijective multiplicative mapping of a Boolean ring
R
onto an arbitrary ring
S
is necessarily additive. We prove a version of Rickart’s theorem for non-bijective mappings. This enables us to partially answer a question that was left open (Al Subaiei, B., Jarboui, N. On the Monoid of Unital Endomorphisms of a Boolean Ring. Axioms 2021, 10, 305).