Are they both fine with your sense of English?
I do not know who is the man standing at the door.
I do not know who the man standing at the door is.

Question

Answer

"I do not know who the man standing at the door is." Is the correct wording for this sentence when you are making a statement about the man at the door. 
You would use "Who is the man standing at the door?" When asking someone a question in order to find out who the man is. 
I hope this helps!

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