What do you put in a formal email's salutation when you don't know who you're writing to (male/female)?

In my point of view, addressing a professional by their designation would be very suitable. For example: Dear Hiring Manager

I would greet whoever it is with a hearty “Hello” or “Good morning”. The key is to make it personal without being too familiar. Make it interesting rather than boring.

Here's something even more important. If you want the recipient to read past “Hello,” make the message all about him or her. Ask yourself, “If I got this email, why should I read it?”

People’s inboxes are crammed with a million messages. Make your message about what he or she wants and you'll stand out in a sea of selfish messages.

Dear Sir / Ma'am

Their designation 

Greetings

than emailbody

Yours Faithfully 

Your Name

hey,, Thanks for invitation :)

usually i start it with "Dear Sir or Mrs" "Hope You Are fine"

and end it with "Best Regards"

in this situations its better to start with... Greetings... hope you are doing well..... its not mandtory to mention dear sir/madum... 

Dear Sir/Madam,... I always use that in such scenarios. (my opinion)

Dear Sir or Madam or "Sir/Madam" (If the gender of the reader is unknown).To Whom It May Concern (If the writer wishes to exclude the gender of the reader from the salutation and/or to convey that the reader should forward the copy to one more suited to receive or respond appropriately).

HELLO,

I think it's best to write Dear Joe Simth  or Dear Jane Smith when you don't know the gender.

My name is Almas Nazeer.

A lot of people write to me as Mr Almas Nazeer. 

Sometimes it is annoying.

I prefer, Dear Almas Nazeer.

If you are writing someone Senior or in respectable post, it would make sense to look them up on Google, you might find their LinkedIn profile.

Thanks

ALMAS

When you dont know who you are writing to its best to you a Dear Sir/ Madam

A situation to be avoided if at all possible.  It's ok to phone and ask to whom an email or letter should be addressed.  If all else fails, the traditional convention is something along the lines of

Dear Madam or Sir,

The order doesn't matter, but the usual way is 'Sir' first so by putting it second you attract the reader's attention and they are more likely to read the message.  Note that this approach is unlikely to be well particularly received by somebody who identifies as non-binary.

Alternatively, put the job title. 

Dear Head of R&D,

Even if you don't know if the recipient firm uses that title, it'll help get the message to the right person.

Amicalement votre et cordialement

I would say a simple Hello or address their full name like Dear Ross Johnson to convey a respectful greeting or introduction.