diff --git a/setup.py b/setup.py

index 80267e8..164cabf 100644

--- a/setup.py

+++ b/setup.py

@@ -26,6 +26,8 @@ prefer_system_libb2 = True

 prefer_system_libxxhash = True

 # prefer_system_msgpack is another option, but you need to set it in src/borg/helpers.py.

+prefer_system_msgpack = bool(os.environ.get('BORG_EXTERNAL_MSGPACK'))

+

 min_python = (3, 5)

 my_python = sys.version_info

@@ -38,6 +40,20 @@ if my_python < min_python:

 on_rtd = os.environ.get('READTHEDOCS')

 install_requires = []

+if prefer_system_msgpack:

+    install_requires.extend([

+        # we are rather picky about msgpack versions, because a good working msgpack is

+        # very important for borg, see https://github.com/borgbackup/borg/issues/3753

+        # best versions seem to be 0.4.6, 0.4.7, 0.4.8 and 0.5.6:

+        'msgpack-python >=0.4.6, <=0.5.6, !=0.5.0, !=0.5.1, !=0.5.2, !=0.5.3, !=0.5.4, !=0.5.5',

+        # if you can't satisfy the above requirement, these are versions that might

+        # also work ok, IF you make sure to use the COMPILED version of msgpack-python,

+        # NOT the PURE PYTHON fallback implementation: ==0.5.1, ==0.5.4

+        #

+        # Please note:

+        # using any other version is not supported by borg development and

+        # any feedback related to issues caused by this will be ignored.

+    ])

 # note for package maintainers: if you package borgbackup for distribution,

 # please add llfuse as a *requirement* on all platforms that have a working

@@ -88,9 +104,9 @@ cython_c_sources = [

 cython_cpp_sources = [

     # these .pyx will get compiled to .cpp

-    msgpack_packer_source,

-    msgpack_unpacker_source,

 ]

+if not prefer_system_msgpack:

+    cython_cpp_sources.extend([msgpack_packer_source, msgpack_unpacker_source])

 try:

     from Cython.Distutils import build_ext

@@ -121,9 +137,12 @@ try:

                 'src/borg/platform/syncfilerange.c',

                 'src/borg/platform/freebsd.c',

                 'src/borg/platform/darwin.c',

-                'src/borg/algorithms/msgpack/_packer.cpp',

-                'src/borg/algorithms/msgpack/_unpacker.cpp',

             ])

+            if not prefer_system_msgpack:

+                self.filelist.extend([

+                    'src/borg/algorithms/msgpack/_packer.cpp',

+                    'src/borg/algorithms/msgpack/_unpacker.cpp',

+                ])

             super().make_distribution()

 except ImportError:

@@ -146,11 +165,15 @@ except ImportError:

     msgpack_packer_source = msgpack_packer_source.replace('.pyx', '.cpp')

     msgpack_unpacker_source = msgpack_unpacker_source.replace('.pyx', '.cpp')

-    from distutils.command.build_ext import build_ext

-    if not on_rtd and not all(os.path.exists(path) for path in [

+    cythonized_sources = [

         compress_source, crypto_ll_source, chunker_source, hashindex_source, item_source, checksums_source,

         platform_posix_source, platform_linux_source, platform_syncfilerange_source, platform_freebsd_source, platform_darwin_source,

-        msgpack_packer_source, msgpack_unpacker_source]):

+    ]

+    if not prefer_system_msgpack:

+        cythonized_sources.extend([msgpack_packer_source, msgpack_unpacker_source])

+

+    from distutils.command.build_ext import build_ext

+    if not on_rtd and not all(os.path.exists(path) for path in cythonized_sources):

         raise ImportError('The GIT version of Borg needs Cython. Install Cython or use a released version.')

@@ -809,26 +832,29 @@ if not on_rtd:

                                                system_prefix=libxxhash_prefix, system=libxxhash_system,

                                                **crypto_ext_kwargs)

-    msgpack_endian = '__BIG_ENDIAN__' if (sys.byteorder == 'big') else '__LITTLE_ENDIAN__'

-    msgpack_macros = [(msgpack_endian, '1')]

-    msgpack_packer_ext_kwargs = dict(

-        sources=[msgpack_packer_source],

-        include_dirs=include_dirs,

-        library_dirs=library_dirs,

-        define_macros=msgpack_macros,

-        language='c++',

-    )

-    msgpack_unpacker_ext_kwargs = dict(

-        sources=[msgpack_unpacker_source],

-        include_dirs=include_dirs,

-        library_dirs=library_dirs,

-        define_macros=msgpack_macros,

-        language='c++',

-    )

+    if not prefer_system_msgpack:

+        msgpack_endian = '__BIG_ENDIAN__' if (sys.byteorder == 'big') else '__LITTLE_ENDIAN__'

+        msgpack_macros = [(msgpack_endian, '1')]

+        msgpack_packer_ext_kwargs = dict(

+            sources=[msgpack_packer_source],

+            include_dirs=include_dirs,

+            library_dirs=library_dirs,

+            define_macros=msgpack_macros,

+            language='c++',

+        )

+        msgpack_unpacker_ext_kwargs = dict(

+            sources=[msgpack_unpacker_source],

+            include_dirs=include_dirs,

+            library_dirs=library_dirs,

+            define_macros=msgpack_macros,

+            language='c++',

+        )

+        ext_modules += [

+            Extension('borg.algorithms.msgpack._packer', **msgpack_packer_ext_kwargs),

+            Extension('borg.algorithms.msgpack._unpacker', **msgpack_unpacker_ext_kwargs),

+        ]

     ext_modules += [

-        Extension('borg.algorithms.msgpack._packer', **msgpack_packer_ext_kwargs),

-        Extension('borg.algorithms.msgpack._unpacker', **msgpack_unpacker_ext_kwargs),

         Extension('borg.compress', **compress_ext_kwargs),

         Extension('borg.crypto.low_level', **crypto_ext_kwargs),

         Extension('borg.hashindex', [hashindex_source]),