Messrs. So & So Company

Presently, when I write a letter to a company (no matter whether the company is "(Name of person) & Co., Ltd." or not, I don't put "Messrs." to the company's name.

Do I have to put "Messrs." before the company's name if the company's name is, for example, "Snappy & Co., Ltd."?

"Messrs. Snappy & Co., Ltd.
Dear Sir or Madam:"

or

"Messrs. Snappy & Co., Ltd.
Dear Sirs (or should I write "Gentlemen" instead?)"

I usually address the company as follows:
"Snappy & Co., Ltd.
Dear Sir or Madam:
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With reference to..."