diff --git a/Cargo.toml b/Cargo.toml
index 52f2662..e8401bc 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -36,7 +36,7 @@ email-encoding = { version = "0.3", optional = true }
 
 # file transport
 uuid = { version = "1", features = ["v4"], optional = true }
-serde = { version = "1", features = ["derive"], optional = true }
+serde = { version = "1.0.110", features = ["derive"], optional = true }
 serde_json = { version = "1", optional = true }
 
 # smtp-transport