There are some slight differences in connotations of each letter ending. I'll order them in increasing formality.
信件的不同结尾会有一些细微的差异。小编把它们依照正式程度从小到大排序。
Love
爱你
Very personal. Used between lovers, family and close friends.
非常私人的口吻，经常使用在爱人，家人和亲密的朋友之间。
Cheers
再见
Very casual. Used between friends. Can be used between coworkers if you want to appear very casual, at the risk of sounding unprofessional.
比较随意的语气，经常使用在朋友之间。如果你想表现的非常随和，也能用在同事之间，但可能有看着不专业的风险。
Take care
保重
Casual and used between friends.
随意，常用于朋友之间。
Thanks or Thank You
谢谢或谢谢你
'Thanks' is very casual and 'Thank You' is formal. Usually used in a business setting. 'Thank You' is a great way to end an email to a stranger, especially if you are making a request or asking a question.
“谢谢”是随意的口吻，“谢谢你”是正式的口吻，常用在商业场合。给陌生人写邮件，“谢谢你”是一个很好的结尾，尤其是当你提出要求或提出问题的时候。
Yours or Yours Truly or Truly
鄙人
Casual or Business casual. A more personal, but still polite way to end the letter. You've probably met the recipient in real life.
休闲或商务休闲口吻。一种更私人，但还是礼貌地结束信件的方式。收件人是你可能已经在现实生活中见过了的。
Best or Best Wishes
（最）美好的祝愿
Casual or Business casual. A polite way to end the letter. Can be used between friends or strangers.
休闲或商务休闲口吻，一种礼貌地结束信件的方式，可以用于朋友或陌生人。
Sincerely or Sincerely Yours or Yours Sincerely
谨启
Casual, Business casual or Formal. The standard letter ending. 'Sincerely' is the most versatile ending because it can be used to end either casual or formal correspondence. If you're unsure about how to sign your letter, use 'Sincerely'.
随意，商务休闲或正式，是一种标准的信件结束语。“忠诚的”是被使用最多的结尾方式，因为它既可以用于结束随意的通信，也可以用于正式通信。如果你不确定如何签署你的信件，那就用“忠诚的”。
Regards or Best Regards
致敬
Business casual or formal. You would use this if the recipient is someone you haven't met in real life.
商务休闲或正式口吻。如果你没有在现实生活中见过收件人，那就用这个。
Cordially
热诚的
Formal. Used in business correspondence when the topic of the letter is serious. Sometimes used if there is a conflict between the sender and the recipient.
正式。在商务信函中，当通信话题比较严肃时，它经常被使用。有时，当寄信人和收件人之间有矛盾时，它也会被使用。
Using an ending that is too casual for the situation may not always be inappropriate. Conversely, using a casual ending can show your self-confidence. But to do this requires you to gauge whether the recipient would like a more casual conversation.
使用一个过于随意的结束语，在某些情况下可能不太合适。相反，使用一个随意的结束语也能展示你的自信。但这样做之前，需要衡量收件人是否喜欢更轻松的对话氛围。
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