sincerely has 3 distinct synonyms spread across 2 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is insincerely.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
with sincerity; without pretense
“she praised him sincerely for his victory”
written formula for ending a letter
Synonyms, antonyms and the broader and narrower relations for sincerely come from Princeton University’s WordNet. Full sources and licences.