"Dear Sir (or Madam)" when gender unknown? [duplicate]

When writing a formal letter addressed to someone whose gender is unknown, how important is it to use "Dear Sir or Madam", instead of "Dear Sir"?

I was taught that "Dear Sir" is an acceptable gender-neutral salutation but times may have changed, is this right or wrong?

Solution 1:

Dear Sir may have been an acceptable gender-neutral salutation some time ago, but I would suggest 'To whom it may concern,' as a more modern and suitable alternative :)

Solution 2:

When the addressee is in the same field as the writer, you can use the address Dear Colleague.

Solution 3:

The proper way to begin a formal letter to someone whose gender is unknown is with the "Dear Sir/Madam," phrase while At the end the letter use the phrase "Yours faithfully," plus your full name, like the template that follows:

Dear Sir/Madam,

....

Yours faithfully, Your full name