diff --git a/pykolab/imap_utf7.py b/pykolab/imap_utf7.py
--- a/pykolab/imap_utf7.py
+++ b/pykolab/imap_utf7.py
@@ -22,58 +22,70 @@
 # OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
 # WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
+# https://twistedmatrix.com/trac/attachment/ticket/6289/imap4.patch
+# #6289 enhancement closed fixed (fixed)
+# Port imap4-utf-7 codec implementation to Python 3
 
 class FolderNameError(ValueError):
     pass
 
 
+try:
+    unicode()
+except:
+    unicode = str
+
+memory_cast = getattr(memoryview, "cast", lambda *x: x[0])
+
 def encode(s):
     if isinstance(s, str) and sum(n for n in (ord(c) for c in s) if n > 127):
         try:
-            s = unicode(s, "UTF-8")
+            s = bytes(s, "UTF-8")
         except Exception:
             raise FolderNameError("%r contains characters not valid in a str folder name. "
-                              "Convert to unicode first?" % s)
+                                  "Convert to unicode first?" % s)
 
-    r = []
+    r = bytearray()
     _in = []
+    valid_chars = set(map(chr, range(0x20, 0x7f))) - {u"&"}
     for c in s:
-        if ord(c) in (range(0x20, 0x26) + range(0x27, 0x7f)):
+        if c in valid_chars:
             if _in:
-                r.extend(['&', modified_base64(''.join(_in)), '-'])
+                r += b'&' + modified_base64(''.join(_in)) + b'-'
                 del _in[:]
-            r.append(str(c))
-        elif c == '&':
+            r.append(ord(c))
+        elif c == u'&':
             if _in:
-                r.extend(['&', modified_base64(''.join(_in)), '-'])
+                r += b'&' + modified_base64(''.join(_in)) + b'-'
                 del _in[:]
-            r.append('&-')
+            r += b'&-'
         else:
             _in.append(c)
     if _in:
-        r.extend(['&', modified_base64(''.join(_in)), '-'])
+        r.extend(b'&' + modified_base64(''.join(_in)) + b'-')
 
-    return ''.join(r)
+    return bytes(r)
 
 
 def decode(s):
     r = []
     decode = []
+    s = memory_cast(memoryview(s), 'c')
     for c in s:
-        if c == '&' and not decode:
-            decode.append('&')
-        elif c == '-' and decode:
+        if c == b'&' and not decode:
+            decode.append(u'&')
+        elif c == b'-' and decode:
             if len(decode) == 1:
-                r.append('&')
+                r.append(u'&')
             else:
-                r.append(modified_unbase64(''.join(decode[1:])))
+                r.append(modified_unbase64(b''.join(decode[1:])))
             decode = []
         elif decode:
             decode.append(c)
         else:
-            r.append(c)
+            r.append(c.decode())
     if decode:
-        r.append(modified_unbase64(''.join(decode[1:])))
+        r.append(modified_unbase64(b''.join(decode[1:])))
     out = ''.join(r)
 
     if not isinstance(out, unicode):
@@ -83,9 +95,9 @@
 
 def modified_base64(s):
     s_utf7 = s.encode('utf-7')
-    return s_utf7[1:-1].replace('/', ',')
+    return s_utf7[1:-1].replace(b'/', b',')
 
 
 def modified_unbase64(s):
-    s_utf7 = '+' + s.replace(',', '/') + '-'
+    s_utf7 = b'+' + s.replace(b',', b'/') + b'-'
     return s_utf7.decode('utf-7')