diff --git a/stdlib/mailbox.pyi b/stdlib/mailbox.pyi
index 4ce792300..c59ee55d3 100644
--- a/stdlib/mailbox.pyi
+++ b/stdlib/mailbox.pyi
@@ -1,6 +1,7 @@
 import email.message
 import sys
 from _typeshed import Self, StrOrBytesPath
+from abc import ABCMeta, abstractmethod
 from types import TracebackType
 from typing import IO, Any, AnyStr, Callable, Generic, Iterable, Iterator, Mapping, Protocol, Sequence, TypeVar, Union, overload
 from typing_extensions import Literal
@@ -27,21 +28,28 @@ class Mailbox(Generic[_MessageT]):
     def __init__(
         self, path: StrOrBytesPath, factory: Callable[[IO[Any]], _MessageT] | None = ..., create: bool = ...
     ) -> None: ...
+    @abstractmethod
     def add(self, message: _MessageData) -> str: ...
+    @abstractmethod
     def remove(self, key: str) -> None: ...
     def __delitem__(self, key: str) -> None: ...
     def discard(self, key: str) -> None: ...
+    @abstractmethod
     def __setitem__(self, key: str, message: _MessageData) -> None: ...
     @overload
     def get(self, key: str, default: None = ...) -> _MessageT | None: ...
     @overload
     def get(self, key: str, default: _T) -> _MessageT | _T: ...
     def __getitem__(self, key: str) -> _MessageT: ...
+    @abstractmethod
     def get_message(self, key: str) -> _MessageT: ...
     def get_string(self, key: str) -> str: ...
+    @abstractmethod
     def get_bytes(self, key: str) -> bytes: ...
     # As '_ProxyFile' doesn't implement the full IO spec, and BytesIO is incompatible with it, get_file return is Any here
+    @abstractmethod
     def get_file(self, key: str) -> Any: ...
+    @abstractmethod
     def iterkeys(self) -> Iterator[str]: ...
     def keys(self) -> list[str]: ...
     def itervalues(self) -> Iterator[_MessageT]: ...
@@ -49,7 +57,9 @@ class Mailbox(Generic[_MessageT]):
     def values(self) -> list[_MessageT]: ...
     def iteritems(self) -> Iterator[tuple[str, _MessageT]]: ...
     def items(self) -> list[tuple[str, _MessageT]]: ...
+    @abstractmethod
     def __contains__(self, key: str) -> bool: ...
+    @abstractmethod
     def __len__(self) -> int: ...
     def clear(self) -> None: ...
     @overload
@@ -58,9 +68,13 @@ class Mailbox(Generic[_MessageT]):
     def pop(self, key: str, default: _T = ...) -> _MessageT | _T: ...
     def popitem(self) -> tuple[str, _MessageT]: ...
     def update(self, arg: _HasIteritems | _HasItems | Iterable[tuple[str, _MessageData]] | None = ...) -> None: ...
+    @abstractmethod
     def flush(self) -> None: ...
+    @abstractmethod
     def lock(self) -> None: ...
+    @abstractmethod
     def unlock(self) -> None: ...
+    @abstractmethod
     def close(self) -> None: ...
     if sys.version_info >= (3, 9):
         def __class_getitem__(cls, item: Any) -> GenericAlias: ...
@@ -71,7 +85,19 @@ class Maildir(Mailbox[MaildirMessage]):
     def __init__(
         self, dirname: StrOrBytesPath, factory: Callable[[IO[Any]], MaildirMessage] | None = ..., create: bool = ...
     ) -> None: ...
+    def add(self, message: _MessageData) -> str: ...
+    def remove(self, key: str) -> None: ...
+    def __setitem__(self, key: str, message: _MessageData) -> None: ...
+    def get_message(self, key: str) -> MaildirMessage: ...
+    def get_bytes(self, key: str) -> bytes: ...
     def get_file(self, key: str) -> _ProxyFile[bytes]: ...
+    def iterkeys(self) -> Iterator[str]: ...
+    def __contains__(self, key: str) -> bool: ...
+    def __len__(self) -> int: ...
+    def flush(self) -> None: ...
+    def lock(self) -> None: ...
+    def unlock(self) -> None: ...
+    def close(self) -> None: ...
     def list_folders(self) -> list[str]: ...
     def get_folder(self, folder: str) -> Maildir: ...
     def add_folder(self, folder: str) -> Maildir: ...
@@ -79,9 +105,20 @@ class Maildir(Mailbox[MaildirMessage]):
     def clean(self) -> None: ...
     def next(self) -> str | None: ...
 
-class _singlefileMailbox(Mailbox[_MessageT]): ...
+class _singlefileMailbox(Mailbox[_MessageT], metaclass=ABCMeta):
+    def add(self, message: _MessageData) -> str: ...
+    def remove(self, key: str) -> None: ...
+    def __setitem__(self, key: str, message: _MessageData) -> None: ...
+    def iterkeys(self) -> Iterator[str]: ...
+    def __contains__(self, key: str) -> bool: ...
+    def __len__(self) -> int: ...
+    def lock(self) -> None: ...
+    def unlock(self) -> None: ...
+    def flush(self) -> None: ...
+    def close(self) -> None: ...
 
 class _mboxMMDF(_singlefileMailbox[_MessageT]):
+    def get_message(self, key: str) -> _MessageT: ...
     def get_file(self, key: str, from_: bool = ...) -> _PartialFile[bytes]: ...
     def get_bytes(self, key: str, from_: bool = ...) -> bytes: ...
     def get_string(self, key: str, from_: bool = ...) -> str: ...
@@ -100,7 +137,19 @@ class MH(Mailbox[MHMessage]):
     def __init__(
         self, path: StrOrBytesPath, factory: Callable[[IO[Any]], MHMessage] | None = ..., create: bool = ...
     ) -> None: ...
+    def add(self, message: _MessageData) -> str: ...
+    def remove(self, key: str) -> None: ...
+    def __setitem__(self, key: str, message: _MessageData) -> None: ...
+    def get_message(self, key: str) -> MHMessage: ...
+    def get_bytes(self, key: str) -> bytes: ...
     def get_file(self, key: str) -> _ProxyFile[bytes]: ...
+    def iterkeys(self) -> Iterator[str]: ...
+    def __contains__(self, key: str) -> bool: ...
+    def __len__(self) -> int: ...
+    def flush(self) -> None: ...
+    def lock(self) -> None: ...
+    def unlock(self) -> None: ...
+    def close(self) -> None: ...
     def list_folders(self) -> list[str]: ...
     def get_folder(self, folder: StrOrBytesPath) -> MH: ...
     def add_folder(self, folder: StrOrBytesPath) -> MH: ...
@@ -113,6 +162,8 @@ class Babyl(_singlefileMailbox[BabylMessage]):
     def __init__(
         self, path: StrOrBytesPath, factory: Callable[[IO[Any]], BabylMessage] | None = ..., create: bool = ...
     ) -> None: ...
+    def get_message(self, key: str) -> BabylMessage: ...
+    def get_bytes(self, key: str) -> bytes: ...
     def get_file(self, key: str) -> IO[bytes]: ...
     def get_labels(self) -> list[str]: ...