Synonyms for Mandatory (same or very similar meaning)
WordNet sense 1 (the recipient of a mandate):
mandatary
WordNet sense 2 (a territory surrendered by Turkey or Germany after World War I and put under the tutelage of some other European power until they are able to stand by themselves):
mandate
WordNet sense 3 (required by rule):
compulsory, required
WordNet sense 4 (morally or legally constraining or binding):
obligatory
From the ODE community, based on WordNet
add/edit