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。接下来我将把它们依照正式程度递增的顺序排序(并逐一解读)。

Love

Very personal. Used between lovers, family and close friends.
语气非常私人化。经用于爱人、家人和亲密的朋友之间。

Cheers

Very casual. Used between friends. Can be used between coworkers if you want to appear very casual, at the risk of sounding unprofessional.
语气非常随意。用于朋友之间。在给同事发邮件时如果你想表现得非常随和,也可以使用,但可能有听起来略不专业的风险。

Take care

Casual and used between friends.
语气随意,用于朋友之间。

Thanks or Thank You

'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.
 “Thanks”是非常随意的语气,而“Thank you”较为正式、常用在商业场合。给陌生人写邮件,尤其是你提出请求或者问问题的时候,“Thank you”是一个很好的邮件结尾。

Yours or Yours Truly or Truly

Casual or Business casual. A more personal, but still polite way to end the letter. You've probably met the recipient in real life.
休闲或商务休闲语气。一种更私人,但还是礼貌的信件落款方式。收件人是你很可能已经在现实生活中见过面的。

Best or Best Wishes

Casual or Business casual. A polite way to end the letter. Can be used between friends or strangers.
休闲或商务休闲语气。一种礼貌的信件落款方式,可以用于朋友或陌生人。

Sincerely or Sincerely Yours or Yours Sincerely

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'.
语气随意、商务休闲或正式。这是一种标准的信件结束语。“Sincerely”是被使用得最多的结尾方式,因为它既可以用于结束较为随意的通信,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“Sincerely”。

Regards or Best Regards

Business casual or formal. You would use this if the recipient is someone you haven't met in real life.
商务休闲或正式语气。如果你没有在现实生活中见过收件人,那就用这个。

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.
语气正式。用于通信话题比较严肃的商务信件中。有时也用于寄信人和收件人之间有矛盾时。

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.
邮件中使用一个过于随意的结束语,在某些情况下可能不太合适。但反过来说,使用一个较为随意的邮件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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