The traditional way to address someone you don’t know in a letter has always been Dear Mr. or Dear Mrs./Ms, but then, what if you don’t know whether the person you are writing to is a man or woman. For example, names like Drew, Jordan or Alex could be man or woman, and of course, you don’t want to cause offence. In this case just use the full name: Dear Alex Jones, or, if you can, do your homework and find out before sending the letter. If you do not know the name of the recipient, you can keep the letter neutral and use Dear Sir or Madam.
If you do know the name of a female recipient, you then have the problem of knowing whether or not she is married, and whether to use Ms. or Mrs. In this age of political correctness, it is best not to presume a woman is married. Twenty years ago, using Mrs. incorrectly would have gotten nothing more than a chuckle, and would have been considered as an attempt of politeness on your behalf. Nowadays, a woman may take offence to the presumption that she is married, or be annoyed that you presumed she was single, even if she is. And therefore, if you aren’t sure, it is best to use the full name of the lady you are writing to: Dear Janet Jones.
If you do know the marital status of the female recipient you are writing to, then Ms./Mrs. Jones is appropriate. But then there is another potential issue; this lady might have a Ph.D., and prefer the title of Dr. Jones, and so again, my advice is to stay as neutral as possible and go with the full name: Dear Janet Jones.
Wow…so much to think about!