Dear Sir Or Madam, how are you?

尊敬的先生，女士们，你们好？

Dear Sir Or Madam.

尊敬的先生或女士。

You can just put "Dear Sir or Madam", and you can write "to whom it may concern".

你可以就写“亲爱的先生或者女士”，也可以写“尊敬的相关人士”。

Dear Sir or Madam: The original and one copy of our leasing agreement1 are enclosed.

简要翻译附上我们租赁合同的原件及一份复印件，请查收。

Dear Sir Or Madam: Thank you very much for the nice comments and your continuous support.

尊敬的客人，您好，非常感谢您的入住及点评！很荣幸饭店的性价比可以被再次肯定。