diff --git a/apps/api/src/dkv/dkv-mail.service.ts b/apps/api/src/dkv/dkv-mail.service.ts
index 60c28fe..de276b5 100644
--- a/apps/api/src/dkv/dkv-mail.service.ts
+++ b/apps/api/src/dkv/dkv-mail.service.ts
@@ -103,6 +103,11 @@ export class DkvMailService {
       );
       // RETHROW — caller (DkvService) handles exponential backoff retries (D-16)
       throw error;
+    } finally {
+      // Always close the transport to release the SMTP connection pool (WR-01).
+      // Without this, repeated sends (especially with retry backoff) accumulate
+      // open connections and can exhaust OS socket limits.
+      transport.close();
     }
   }
 }