Fix interoperability of rfc822_escape with stblib's email library

distutils/util.py

@@ -508,6 +508,12 @@ def rfc822_escape(header):
     """Return a version of the string escaped for inclusion in an
     RFC-822 header, by ensuring there are 8 spaces space after each newline.
     """
-    lines = header.split('\n')
-    sep = '\n' + 8 * ' '
-    return sep.join(lines)
+    indent = 8 * " "
+    lines = header.splitlines(keepends=True)
+
+    # Emulate the behaviour of `str.split`
+    # (the terminal line break in `splitlines` does not result in an extra line):
+    ends_in_newline = lines and lines[-1].splitlines()[0] != lines[-1]
+    suffix = indent if ends_in_newline else ""
+
+    return indent.join(lines) + suffix