diff --git a/apps/api/src/docker/runner.ts b/apps/api/src/docker/runner.ts
index 7ae7c7b..c665401 100644
--- a/apps/api/src/docker/runner.ts
+++ b/apps/api/src/docker/runner.ts
@@ -95,7 +95,11 @@ export const runDocker = async (
       throw new HttpError(504, "Docker Mailserver command timed out.");
     }
 
-    throw new HttpError(502, "Docker Mailserver is unreachable.");
+    if (error instanceof Error && "code" in error && error.code === "ENOENT") {
+      throw new HttpError(502, `Docker binary '${config.dockerBinary}' was not found. Set DOCKER_BINARY to the absolute docker path on NixOS.`);
+    }
+
+    throw new HttpError(502, error instanceof Error ? error.message : "Docker Mailserver is unreachable.");
   } finally {
     clearTimeout(timeout);
   }