Your are reading the documentation wrong.  For linesep, it says "The string to be used to terminate lines in serialized output."  The key word there is "output".  email operates in "universal newline mode" when parsing messages, turning all line endings into python's internal \n format.  What the linesep policy controls is the *output* line separator.

Note that the python2 behavior for decode=True is arguably a bug, but not one we are going to fix for backward compatibility reasons.