https://www.english-at-home.com/business/how-to-start-and-end-a-business-english-letter-or-email/
https://site.uit.no/english/writing-style/letters/
https://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv153.shtml
https://web.mit.edu/course/21/21.guide/letters.htm
