sin·cere
sinˈsir/
adjective
free from pretense or deceit; proceeding from genuine feelings.
"they offer their sincere thanks to Paul"
synonyms:heartfelt, wholehearted, profound, deep;More
(of a person) saying what they genuinely feel or believe; not dishonest or hypocritical.
if I were you I'd probably use a formal format i.e. sir or madam
