If you have to send an email to someone acquaintance and you only know the first name, would you address him/her as Mr. John or Ms. Jane? The intention is to sound as formal as possible. So, no 'Hello John/Jane'
I think this answer will depend very much on exactly what culture you're in; I can only address the United States.
In mainstream US culture, you don't say Mr./Mrs./Ms. Firstname. Titles like Mr. or Mrs. are only used with last names.
If you are trying to be as formal as possible, you don't use someone's first name, so it doesn't matter whether you know it or not. Someone whose last name you don't know can be addressed in mail as Dear Sir or Dear Madam.
Now for a side note:
In some US subcultures, Mr./Mrs./Ms. Firstname is used, but only semi-formally. You would use that form of address for someone you were familiar with and wanted to address respectfully (like someone older), but not a stranger. For example, my wife is from the Deep South of the US, and before we were married, her nephew called me Mr. Firstname, and my wife calls her friend's mother Mrs. Firstname. There are some good discussions of the practice at Southern U.S. use of "Miss FirstName" -- racial or class connotations? and Can “Mr”, “Mrs”, etc. be used with a first name?
In the northeastern US where I grew up, this form of address is not used.