diff --git a/src/govoplan_calendar/backend/outbox.py b/src/govoplan_calendar/backend/outbox.py
index c31ebce..d59a9c2 100644
--- a/src/govoplan_calendar/backend/outbox.py
+++ b/src/govoplan_calendar/backend/outbox.py
@@ -858,15 +858,18 @@ def _fail_operation(
     session.flush()
 
 
-def execute_calendar_outbox_operation(
+def _lock_leased_outbox_operation(
     session: Session,
     *,
     operation_id: str,
     lease_token: str,
-    client_factory: Callable[[Session, CalendarSyncSource], object] | None = None,
-) -> CalendarOutboxOperation:
+) -> tuple[CalendarOutboxOperation, CalendarSyncSource | None]:
     operation = session.get(CalendarOutboxOperation, operation_id)
-    if operation is None or operation.status != "in_progress" or operation.lease_token != lease_token:
+    if (
+        operation is None
+        or operation.status != "in_progress"
+        or operation.lease_token != lease_token
+    ):
         raise ValueError("Calendar outbox lease is no longer valid")
     source = (
         session.query(CalendarSyncSource)
@@ -882,151 +885,282 @@ def execute_calendar_outbox_operation(
     )
     if operation.status != "in_progress" or operation.lease_token != lease_token:
         raise ValueError("Calendar outbox lease is no longer valid")
-    operation_metadata = operation.metadata_ or {}
-    unavailable_reason = _operation_source_unavailable_reason(operation, source)
-    if unavailable_reason:
-        if source is not None and source.tenant_id == operation.tenant_id:
-            _fail_operation(
-                session,
-                operation=operation,
-                source=source,
-                error=unavailable_reason,
-                terminal_status="cancelled",
-            )
-        else:
-            operation.status = "cancelled"
-            operation.completed_at = utcnow()
-            operation.last_error = unavailable_reason
-            operation.lease_token = None
-            operation.lease_expires_at = None
-            session.flush()
-        return operation
+    return operation, source
 
-    overwrite = bool(operation_metadata.get("overwrite")) if isinstance(operation_metadata, dict) else False
-    client: object | None = None
-    try:
-        if client_factory is None:
-            from govoplan_calendar.backend.service import caldav_client_for_source
 
-            client = caldav_client_for_source(session, source)
-        else:
-            client = client_factory(session, source)
-        if operation.operation_kind == "put":
-            result = client.put_object(
-                operation.resource_href,
-                operation.payload_ics or "",
-                etag=operation.expected_etag,
-                create=not bool(operation.expected_etag),
-                overwrite=overwrite,
-            )
-            remote_etag = result.etag
-            reconciled = False
-            if not remote_etag:
-                try:
-                    matched, remote_etag = _remote_matches_operation(client, operation)
-                except Exception:
-                    matched, remote_etag = False, None
-                if not matched or not remote_etag:
-                    _fail_operation(
-                        session,
-                        operation=operation,
-                        source=source,
-                        error="CalDAV PUT succeeded without an ETag and could not be reconciled safely",
-                    )
-                    return operation
-                reconciled = True
-            _complete_operation(
-                session,
-                operation=operation,
-                source=source,
-                remote_etag=remote_etag,
-                reconciled=reconciled,
-            )
-            return operation
-
-        if not operation.expected_etag and not overwrite:
-            matched, remote_etag = _remote_matches_operation(client, operation)
-            if matched:
-                _complete_operation(
-                    session,
-                    operation=operation,
-                    source=source,
-                    remote_etag=remote_etag,
-                    reconciled=True,
-                )
-                return operation
-            _fail_operation(
-                session,
-                operation=operation,
-                source=source,
-                error=(
-                    "CalDAV DELETE has no known ETag and the remote resource exists; "
-                    "refusing to delete an unknown remote version"
-                ),
-                terminal_status="conflict",
-            )
-            return operation
-        result = client.delete_object(
-            operation.resource_href,
-            etag=operation.expected_etag,
-            overwrite=overwrite,
+def _cancel_undeliverable_operation(
+    session: Session,
+    *,
+    operation: CalendarOutboxOperation,
+    source: CalendarSyncSource | None,
+    reason: str,
+) -> None:
+    if source is not None and source.tenant_id == operation.tenant_id:
+        _fail_operation(
+            session,
+            operation=operation,
+            source=source,
+            error=reason,
+            terminal_status="cancelled",
         )
+        return
+    operation.status = "cancelled"
+    operation.completed_at = utcnow()
+    operation.last_error = reason
+    operation.lease_token = None
+    operation.lease_expires_at = None
+    session.flush()
+
+
+def _calendar_outbox_client(
+    session: Session,
+    *,
+    source: CalendarSyncSource,
+    client_factory: Callable[[Session, CalendarSyncSource], object] | None,
+) -> object:
+    if client_factory is not None:
+        return client_factory(session, source)
+    from govoplan_calendar.backend.service import caldav_client_for_source
+
+    return caldav_client_for_source(session, source)
+
+
+def _try_remote_match(
+    client: object,
+    operation: CalendarOutboxOperation,
+) -> tuple[bool, str | None] | None:
+    try:
+        return _remote_matches_operation(client, operation)
+    except Exception:
+        return None
+
+
+def _complete_if_remote_match_is_usable(
+    session: Session,
+    *,
+    operation: CalendarOutboxOperation,
+    source: CalendarSyncSource,
+    remote_match: tuple[bool, str | None] | None,
+) -> bool:
+    if remote_match is None:
+        return False
+    matched, remote_etag = remote_match
+    if not matched or (operation.operation_kind != "delete" and not remote_etag):
+        return False
+    _complete_operation(
+        session,
+        operation=operation,
+        source=source,
+        remote_etag=remote_etag,
+        reconciled=True,
+    )
+    return True
+
+
+def _execute_calendar_outbox_put(
+    session: Session,
+    *,
+    operation: CalendarOutboxOperation,
+    source: CalendarSyncSource,
+    client: object,
+    overwrite: bool,
+) -> None:
+    result = client.put_object(
+        operation.resource_href,
+        operation.payload_ics or "",
+        etag=operation.expected_etag,
+        create=not bool(operation.expected_etag),
+        overwrite=overwrite,
+    )
+    remote_etag = result.etag
+    if remote_etag:
         _complete_operation(
             session,
             operation=operation,
             source=source,
-            remote_etag=result.etag,
-            reconciled=result.status == 404,
+            remote_etag=remote_etag,
+            reconciled=False,
+        )
+        return
+    remote_match = _try_remote_match(client, operation)
+    if _complete_if_remote_match_is_usable(
+        session,
+        operation=operation,
+        source=source,
+        remote_match=remote_match,
+    ):
+        return
+    _fail_operation(
+        session,
+        operation=operation,
+        source=source,
+        error="CalDAV PUT succeeded without an ETag and could not be reconciled safely",
+    )
+
+
+def _execute_calendar_outbox_delete(
+    session: Session,
+    *,
+    operation: CalendarOutboxOperation,
+    source: CalendarSyncSource,
+    client: object,
+    overwrite: bool,
+) -> None:
+    if not operation.expected_etag and not overwrite:
+        remote_match = _remote_matches_operation(client, operation)
+        if _complete_if_remote_match_is_usable(
+            session,
+            operation=operation,
+            source=source,
+            remote_match=remote_match,
+        ):
+            return
+        _fail_operation(
+            session,
+            operation=operation,
+            source=source,
+            error=(
+                "CalDAV DELETE has no known ETag and the remote resource exists; "
+                "refusing to delete an unknown remote version"
+            ),
+            terminal_status="conflict",
+        )
+        return
+    result = client.delete_object(
+        operation.resource_href,
+        etag=operation.expected_etag,
+        overwrite=overwrite,
+    )
+    _complete_operation(
+        session,
+        operation=operation,
+        source=source,
+        remote_etag=result.etag,
+        reconciled=result.status == 404,
+    )
+
+
+def _handle_outbox_precondition_failure(
+    session: Session,
+    *,
+    operation: CalendarOutboxOperation,
+    source: CalendarSyncSource,
+    client: object | None,
+    error: CalDAVPreconditionFailed,
+) -> None:
+    remote_match = _try_remote_match(client, operation) if client is not None else None
+    if _complete_if_remote_match_is_usable(
+        session,
+        operation=operation,
+        source=source,
+        remote_match=remote_match,
+    ):
+        return
+    if remote_match is None:
+        _fail_operation(session, operation=operation, source=source, error=error)
+        return
+    matched, _remote_etag = remote_match
+    _fail_operation(
+        session,
+        operation=operation,
+        source=source,
+        error=(
+            "Matching CalDAV resource has no usable ETag; retrying reconciliation"
+            if matched
+            else "CalDAV resource changed remotely; reconcile or sync before retrying"
+        ),
+        terminal_status=None if matched else "conflict",
+    )
+
+
+def _handle_outbox_delivery_failure(
+    session: Session,
+    *,
+    operation: CalendarOutboxOperation,
+    source: CalendarSyncSource,
+    client: object | None,
+    error: Exception,
+) -> None:
+    remote_match = _try_remote_match(client, operation) if client is not None else None
+    if _complete_if_remote_match_is_usable(
+        session,
+        operation=operation,
+        source=source,
+        remote_match=remote_match,
+    ):
+        return
+    _fail_operation(session, operation=operation, source=source, error=error)
+
+
+def execute_calendar_outbox_operation(
+    session: Session,
+    *,
+    operation_id: str,
+    lease_token: str,
+    client_factory: Callable[[Session, CalendarSyncSource], object] | None = None,
+) -> CalendarOutboxOperation:
+    operation, source = _lock_leased_outbox_operation(
+        session,
+        operation_id=operation_id,
+        lease_token=lease_token,
+    )
+    unavailable_reason = _operation_source_unavailable_reason(operation, source)
+    if unavailable_reason:
+        _cancel_undeliverable_operation(
+            session,
+            operation=operation,
+            source=source,
+            reason=unavailable_reason,
         )
         return operation
-    except CalDAVPreconditionFailed as exc:
-        if client is None:
-            _fail_operation(session, operation=operation, source=source, error=exc)
-            return operation
-        try:
-            matched, remote_etag = _remote_matches_operation(client, operation)
-        except Exception:
-            _fail_operation(session, operation=operation, source=source, error=exc)
-        else:
-            if matched and (operation.operation_kind == "delete" or remote_etag):
-                _complete_operation(
-                    session,
-                    operation=operation,
-                    source=source,
-                    remote_etag=remote_etag,
-                    reconciled=True,
-                )
-            else:
-                _fail_operation(
-                    session,
-                    operation=operation,
-                    source=source,
-                    error=(
-                        "Matching CalDAV resource has no usable ETag; retrying reconciliation"
-                        if matched
-                        else "CalDAV resource changed remotely; reconcile or sync before retrying"
-                    ),
-                    terminal_status=None if matched else "conflict",
-                )
-        return operation
-    except Exception as exc:
-        matched, remote_etag = False, None
-        if client is not None:
-            try:
-                matched, remote_etag = _remote_matches_operation(client, operation)
-            except Exception:
-                matched, remote_etag = False, None
-        if matched and (operation.operation_kind == "delete" or remote_etag):
-            _complete_operation(
+
+    assert source is not None
+    operation_metadata = operation.metadata_ or {}
+    overwrite = (
+        bool(operation_metadata.get("overwrite"))
+        if isinstance(operation_metadata, dict)
+        else False
+    )
+    client: object | None = None
+    try:
+        client = _calendar_outbox_client(
+            session,
+            source=source,
+            client_factory=client_factory,
+        )
+        if operation.operation_kind == "put":
+            _execute_calendar_outbox_put(
                 session,
                 operation=operation,
                 source=source,
-                remote_etag=remote_etag,
-                reconciled=True,
+                client=client,
+                overwrite=overwrite,
             )
         else:
-            _fail_operation(session, operation=operation, source=source, error=exc)
-        return operation
+            _execute_calendar_outbox_delete(
+                session,
+                operation=operation,
+                source=source,
+                client=client,
+                overwrite=overwrite,
+            )
+    except CalDAVPreconditionFailed as exc:
+        _handle_outbox_precondition_failure(
+            session,
+            operation=operation,
+            source=source,
+            client=client,
+            error=exc,
+        )
+    except Exception as exc:
+        _handle_outbox_delivery_failure(
+            session,
+            operation=operation,
+            source=source,
+            client=client,
+            error=exc,
+        )
+    return operation
 
 
 def dispatch_calendar_outbox(