[adverb]I'm sincerely (= honestly or without falseness) grateful for all the help you've given me.I disagree with my brother's religious beliefs, but they are sincerely held.(Yours) sincerely/(US) Sincerely (yours) is a common way of ending a formal letter which is addressed to a named person.
SINCERELY
Meaning of SINCERELY in English
Cambridge English vocab. Кембриджский английский словарь. 2012