When you are writing a formal email, how do you start when you don't know who you are writing for?

Dear Sir/s . is a better beginning for unknown receiver in a formal mail...  

Thanks Dear,

You can use the following :

• Dear Gents.
• Dears.
• Dear HR , PR, Sales, Marketing ; management.
• Dear Mr. , Mrs.

In such a scenario, I generally say simply "Greetings!" and then proceed with who am I and what I want from them and then complete with thanks. Thanks.

Greetings is always a safe start.

I would say: Dear Sir/Madam or you may use job title - if you know of course.

to whom it may concern

to Sales Manger, Purchasing manager ,Logistic Manager

to all X employess

Dear Sir/s , Madam/s

If it's a company you are writing to, you may start with,

- "Dear Sirs". Note the 's' after Sir. It's the formal way to address both genders. DO NOT WRITE, 'Dear Sir/madam'.

If you are applying for a job online, say:

- "Dear Hiring Manager"

If you are formally addressing a female, say:

- "Dear Ms...." (if you don't know her civil status or if married)

- "Dear Miss..." if you are aware she is single

If you know the person, say

- "Dear Mr. _______" or "Dear Ms. _______" (address the person by his/her surname) 

DO NOT EVER USE "To whom it may concern". This is impolite and impersonal.

I cannot fully answer your question without knowing the content you wish to convey.

But "Dear Sir/Madam" this a good start 

I fully agree with the answers been added by EXPERTS.........Thanks.

Greetings from so and so and your company name  and profession what you are working in the company , and introduce your self to concern