https://www.english-at-home.com/business/how-to-start-and-end-a-business-english-letter-or-email/
https://www.english-at-home.com/business/how-to-start-and-end-a-business-english-letter-or-email/
http://www.nvtc.ee/e-oppe/Varkki/layout/opening_and_closing_lines.html
https://site.uit.no/english/writing-style/letters/
https://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv153.shtml
https://www.quora.com/What-do-you-write-instead-of-sincerely
https://www.quora.com/What-are-the-good-closing-sentences-in-a-formal-letter