dear Sir

• dear sir
• dear Sir or Madam, dear Sir/Madam (if gender of addressee is unknown)
• dear Sirs, dear sirs (with multiple addressees)
• Dear Sirs or Madams (if gender of multiple addressees is unknown)

dear Sir

1. (dated) Used as the salutation at the beginning of a formal letter when addressing persons – assumed to be male – of unknown names.

   Dear Sir, You will recently have received...

Usually, both words will be capitalized, the first because it is the first letter of a sentence, the second because it is used as a title of respect.

• arrides, raiders