Is it correct to start a cover letter like this:
"Dear Sir / Madam..." with a forward slash,
or would you rather use
"Dear Sir or Madam..."
when you don't know your recipient's name?
"Dear Sir or Madam," is the standard form. When not addressing someone by name, end with "Yours faithfully," not "Yours sincerely,". However, it is not too difficult to find out the names of people that you are writing to and you should certainly do this if you are making a job application or if you are writing an important business letter.
I wrote several cover letters and usually started them with “To whom it may concern.” Not the best choice, frankly speaking. That’s a total cliche. “Dear Sir/Madam” is a little bit better. But after I researched this topic myself, I found this page https://www.getcoverletter.com/blog/how-to-address-a-cover-letter/ with some practical advice on addressing the cover letter.
In recognition of the modern aversion to assigning gender to persons unknown, I have had recommended to me to use non-binary alternatives to the more old fashioned Dear Sir/Madam. If you know the job title of the person, in your example when applying for a job you might use:
To the hiring personnel... or Dear Recruitment Coordinator,...