You should know the name, so "Dear John," is the only appropriate salutation.
Business Letter, informal, such as an invitation to an event.
"Dear John," if you have a strong personal relationship.
"Dear John Smith," if you are an acquaintance.
"Dear Mr. Smith," if the person is in a senior position.
"Dear Principal Smith," if the person is a senior administrator, in this example principal of your child's school.
"Dear Claims Adjustor," if you do not know the name.
"Dear Claims Department," if you do not know the position, or are inviting the entire department.
"Dear John Smith Incorporated," if you are inviting the entire company.
Formal business Letter
"Dear John Smith:"
Use a colon for formal business, a comma for personal. The above salutations with a colon would be appropriate.
To whom it may concern:
Do not use "To whom it may concern:" other than for a legal letter, and this would be rare in itself; mainly it would be used for some kind of warning to desist or notice that is required, but the recipient is completely unknown. Possibly a noise complaint to an untraceable occupant. As much as possible include some detail, such as "Occupant, 1234 Oak Street:"
"To whom it may concern" puts the onus on whoever opens the letter to read it and find out where it should be forwarded. Unless your intent is to be rude and troublesome, it does not add anything positive to your letter. Addressing the letter to a job title would be preferred.