diff --git a/src/lib.rs b/src/lib.rs
index 6bc4128..0fa1d3b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -110,7 +110,9 @@
     trivial_numeric_casts,
     unstable_features,
     unused_import_braces,
-    rust_2018_idioms
+    rust_2018_idioms,
+    clippy::string_add,
+    clippy::string_add_assign
 )]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
diff --git a/src/message/dkim.rs b/src/message/dkim.rs
index 1aeeb24..edb9411 100644
--- a/src/message/dkim.rs
+++ b/src/message/dkim.rs
@@ -349,9 +349,10 @@ pub(super) fn dkim_sign(message: &mut Message, dkim_config: &DkimConfig) {
         &dkim_header,
         dkim_config.canonicalization.header,
     );
-    let to_be_signed = signed_headers + &canonicalized_dkim_header;
-    let to_be_signed = to_be_signed.trim_end();
-    let hashed_headers = Sha256::digest(to_be_signed.as_bytes());
+    let mut hashed_headers = Sha256::new();
+    hashed_headers.update(signed_headers.as_bytes());
+    hashed_headers.update(canonicalized_dkim_header.trim_end().as_bytes());
+    let hashed_headers = hashed_headers.finalize();
     let signature = match &dkim_config.private_key.0 {
         InnerDkimSigningKey::Rsa(private_key) => base64::encode(
             private_key