sincerely vs truly

sincerely and truly both are adverbs.

Word	Noun	Adjective	Verb	Adverb
sincerely	No	No	No	Yes
truly	No	No	No	Yes

As adverbs, sincerely and truly are synonyms defined as:

Other synonyms of sincerely include unfeignedly.

sincerely (adverb)	truly (adverb)
with sincerity; without pretense	in accordance with truth or fact or reality
written formula for ending a letter	by right
	with sincerity; without pretense
	in fact (used as intensifiers or sentence modifiers)