security: harden Campaign delivery effects and reconciliation

src/govoplan_campaign/backend/messages/builder.py

@@ -590,6 +590,25 @@ def _validate_required_recipients(
     return MessageValidationStatus.EXCLUDED
 
 
+def _validate_required_sender(
+    senders: list[RecipientConfig],
+    issues: list[MessageIssue],
+    validation_status: MessageValidationStatus,
+) -> MessageValidationStatus:
+    if senders:
+        return validation_status
+    issues.append(
+        MessageIssue(
+            severity="error",
+            code="missing_sender",
+            message="No effective From address is configured; Campaign must resolve the sender before building.",
+            behavior="block",
+            source="recipients",
+        )
+    )
+    return MessageValidationStatus.BLOCKED
+
+
 def _render_message_template(
     config: CampaignConfig,
     campaign_file: str | Path,
@@ -784,6 +803,11 @@ def build_entry_message(
     if not entry.active:
         return _inactive_entry_message(config=config, entry=entry, entry_index=entry_index, context=context)
 
+    context.validation_status = _validate_required_sender(
+        context.senders,
+        context.issues,
+        context.validation_status,
+    )
     context.validation_status = _validate_required_recipients(
         config,
         context.recipients,