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, I'm writing in response to the advertisement you placed in Chicago Newsdaily for a librarian on Monday, Aug. 23.
我是看到你们8月23日星期一在《芝加哥新闻日报》上招聘图书管理员的广告后才写此信的。
Dear Sir Or Madam, as a regular passenger of your train, I am completely disappointed to find that the service you provide is far from satisfactory.
尊敬的先生或女士，作为贵列车的一位常客，我很失望地发现贵方提供的服务远远不能令人满意。
Note that in UK English if you start with Dear Sir or Dear Madam, you should end with Yours faithfully.
要注意在英式英语中，如果你的邮件以以“尊敬的先生”或”尊敬的女士”开头，那应该以”你忠诚的……“结束。
Dear Sir or Madam, Dear teachers, boys and girls, and Dear Grandpas, grandmas, Uncle, Antie.
尊敬的领导，亲爱的老师、同学们以及在座的的各位爷爷奶奶叔叔阿姨。
Dear Sir Or Madam, how are you?
尊敬的先生，女士们，你们好？
P. O. Box 745, Beijing Dear Sir or Madam: [1] I have read your advertisement in yesterday's China Daily. I feel I will be fit for the job needed in your company.
参考译文】北京市745信箱尊敬的先生/女士：我在昨天的《中国日报》上看到贵公司的招聘广告，我认为我很适合贵公司的这个职位。
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。
尊敬的先生或女士。
Dear Sir or Madam, I would like to request information about your company to facilitate my research for a school project.
我想向贵公司咨询一些信息，以帮助我进行一个项目研究。
Dear Sir Or Madam, I am writing to ask for resignation, I want to leave my position next month.
我将于下个月离开贵杂志社，特此向您提出辞职申请。
Dear Sir Or Madam, I am writing to ask for resignation, I want to leave my position next month.
我将于下个月离开贵杂志社，特此向您提出辞职申请。
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