If you want to use their last name, you should continue to include 'Mr', 'Ms', or 'Mrs' as appropriate. "Hi" is rather informal so keep "Hello" in mind as an alternative. Of course if you really don't need to be formal you can just address them by their first name.
Hello, Mr. [LastName]. <- Most formal of these options (Probably best for official email)
Hi, Ms. [LastName].
Hi, [FirstName]. <- Least formal.
For grammar purposes: A last name should usually have either the person's title (Dr, Prof, etc) or the honorific (Mr, Ms, etc). The title takes precedence.
While unknown and unfamiliar both mean the same in this context, I think, the latter one has some flair of being personalized. If you say 'unknown', I'm afraid it may be generalized.
Maybe, because 'unfamiliar' has come from the word 'family', someone's family?
Taking a human example makes it clearer --It's possible that I know Sam, but I'm not 'familiar' with him. I don't know how he is. In other words, I know Sam, he's Kelly's brother; but, I'm not familiar with him; so whether you do business with him, it's your take. Don't consider my opinion on that!
Also, if something is 'unknown', it might be unknown to the world; but if something is 'unfamiliar', you immediately make things personal? Unfamiliar to someone?
I'm visiting an unknown city ~ "Oh really, which corner of the world it is in?"
over...
I'm visiting an unfamiliar city ~ I see, better read it on WikiTravel and go.
This makes me think. If you are visiting 'unknown' place, you may not find any information about it, but if you are unfamiliar with some place, you may have some information. I'm not familiar with New York City, but The Big Apple is not unknown to me!
But again, as I see, both the words have been used in this context. My answer is just a try to get subtlety of it.
But yes, if you define that it's unknown to me, it works!
Best Answer
Just write Samara, for postal addresses.
Unless the official name of your city is actually "City of Samara" - e.g. in the US you have "Oklahoma City" which is the actual full name of the city.
Postal addresses have expected data in specific slots, e.g. for US postal addresses:
So it is usually known you mean a city already by where you put it in the address.