diff --git a/src/govoplan_core/core/postbox.py b/src/govoplan_core/core/postbox.py
index 3b11e27..06a3ffc 100644
--- a/src/govoplan_core/core/postbox.py
+++ b/src/govoplan_core/core/postbox.py
@@ -172,6 +172,8 @@ class PostboxDirectoryEntryRef:
     holder_count: int = 0
     vacant: bool = True
     access: PostboxAccessDecisionRef | None = None
+    resource_revision: int = 1
+    etag: str | None = None
 
 
 @dataclass(frozen=True, slots=True)
@@ -213,6 +215,8 @@ class PostboxMessageRef:
     producer_module: str | None = None
     producer_resource_type: str | None = None
     producer_resource_id: str | None = None
+    in_reply_to_message_id: str | None = None
+    replaces_message_id: str | None = None
     encryption_profile: str = "plaintext_v1"
     key_epoch: int = 1
     ciphertext_ref: str | None = None
@@ -222,6 +226,17 @@ class PostboxMessageRef:
     metadata: Mapping[str, object] = field(default_factory=dict)
 
 
+@dataclass(frozen=True, slots=True)
+class PostboxMessageAuthoringRequest:
+    idempotency_key: str
+    subject: str
+    body_text: str | None = None
+    classification: str = "internal"
+    participants: tuple[PostboxParticipantRef, ...] = ()
+    attachments: tuple[PostboxAttachmentRef, ...] = ()
+    metadata: Mapping[str, object] = field(default_factory=dict)
+
+
 @dataclass(frozen=True, slots=True)
 class PostboxDeliveryReceiptSummaryRef:
     delivery_id: str
@@ -380,6 +395,28 @@ class PostboxMessagesProvider(Protocol):
     ) -> PostboxMessageRef:
         ...
 
+    def create_message(
+        self,
+        session: object,
+        *,
+        tenant_id: str,
+        postbox_id: str,
+        actor: PostboxActorRef,
+        request: PostboxMessageAuthoringRequest,
+    ) -> PostboxMessageRef:
+        ...
+
+    def reply_to_message(
+        self,
+        session: object,
+        *,
+        tenant_id: str,
+        message_id: str,
+        actor: PostboxActorRef,
+        request: PostboxMessageAuthoringRequest,
+    ) -> PostboxMessageRef:
+        ...
+
 
 @runtime_checkable
 class PostboxDeliveryProvider(Protocol):
diff --git a/tests/test_postbox_contract.py b/tests/test_postbox_contract.py
index d55e45a..48db84b 100644
--- a/tests/test_postbox_contract.py
+++ b/tests/test_postbox_contract.py
@@ -63,6 +63,14 @@ class _PostboxProvider:
         del session, tenant_id, message_id, actor, state
         return None
 
+    def create_message(self, session, *, tenant_id, postbox_id, actor, request):
+        del session, tenant_id, postbox_id, actor, request
+        return None
+
+    def reply_to_message(self, session, *, tenant_id, message_id, actor, request):
+        del session, tenant_id, message_id, actor, request
+        return None
+
     def deliver(self, session, request):
         del session, request
         return None