The prefix "un-" means not or no.
Example: Unidentified- not identified.
We can argue that Dickinson sees death here as nothing but guide to eternity.
In the poem Death is not sinister nor scary, instead its a courteous and patient gentlemen that is there to guide the narrator even though she had not time for it. He accompany's her until its her time and though then things get bit spooky it is worth it as in the fourth stanza she arrives at her destination which is eternity. She relishes that death is not death, but immortality.
Well, yeah because, sentences are supposed to be read forward. Reading it backwards, makes no sense.
