adj. 25B6; adjective
our sincere gratitude : HEARTFELT, wholehearted, profound, deep; genuine, real, unfeigned, unaffected, true, honest, bona fide.
a sincere person : HONEST, genuine, truthful, unhypocritical, straightforward, direct, frank, candid; informal straight, upfront, on the level; N. Amer. informal on the up and up.