diff --git a/stdlib/imaplib.pyi b/stdlib/imaplib.pyi
index 54760dd036ca..f13e1c9b656c 100644
--- a/stdlib/imaplib.pyi
+++ b/stdlib/imaplib.pyi
@@ -1,7 +1,7 @@
 import subprocess
 import sys
 import time
-from _typeshed import ReadableBuffer, Self
+from _typeshed import ReadableBuffer, Self, _BufferWithLen
 from builtins import list as _list  # conflicts with a method named "list"
 from collections.abc import Callable
 from datetime import datetime
@@ -151,11 +151,11 @@ class IMAP4_stream(IMAP4):
         def open(self, host: str | None = ..., port: int | None = ...) -> None: ...
 
 class _Authenticator:
-    mech: Callable[[bytes], bytes]
-    def __init__(self, mechinst: Callable[[bytes], bytes]) -> None: ...
+    mech: Callable[[bytes], bytes | bytearray | memoryview | str | None]
+    def __init__(self, mechinst: Callable[[bytes], bytes | bytearray | memoryview | str | None]) -> None: ...
     def process(self, data: str) -> str: ...
-    def encode(self, inp: bytes) -> str: ...
-    def decode(self, inp: str) -> bytes: ...
+    def encode(self, inp: bytes | bytearray | memoryview) -> str: ...
+    def decode(self, inp: str | _BufferWithLen) -> bytes: ...
 
 def Internaldate2tuple(resp: ReadableBuffer) -> time.struct_time | None: ...
 def Int2AP(num: SupportsAbs[SupportsInt]) -> bytes: ...