Official ending of a letter

https://www.english-at-home.com/business/how-to-start-and-end-a-business-english-letter-or-email/

https://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv153.shtml

https://site.uit.no/english/writing-style/letters/

https://english.stackexchange.com/questions/11101/is-there-a-name-for-the-final-section-of-a-letter/11115

https://www.grammar-monster.com/lessons/yours_faithfully_or_sincerely_at_end_of_letter.htm
http://www.blairenglish.com/exercises/emails/exercises/formal-writing-vocabulary-list/words/starting-and-ending-phrases-formal-email.html