diff --git a/lettre_email/src/lib.rs b/lettre_email/src/lib.rs
index bbe5cba..51007f0 100644
--- a/lettre_email/src/lib.rs
+++ b/lettre_email/src/lib.rs
@@ -131,7 +131,7 @@ impl PartBuilder {
 
     /// Adds a `ContentType` header with the given MIME type
     pub fn content_type(self, content_type: &Mime) -> PartBuilder {
-        self.header(("Content-Type", format!("{}", content_type).as_ref()))
+        self.header(("Content-Type", content_type.to_string()))
     }
 
     /// Adds a child part
@@ -303,7 +303,7 @@ impl EmailBuilder {
             .body(body)
             .header((
                 "Content-Type",
-                format!("{}", mime::TEXT_PLAIN_UTF_8).as_ref(),
+                mime::TEXT_PLAIN_UTF_8.to_string(),
             ))
             .build();
         self.child(text)
@@ -315,7 +315,7 @@ impl EmailBuilder {
             .body(body)
             .header((
                 "Content-Type",
-                format!("{}", mime::TEXT_HTML_UTF_8).as_ref(),
+                mime::TEXT_HTML_UTF_8.to_string(),
             ))
             .build();
         self.child(html)
@@ -331,7 +331,7 @@ impl EmailBuilder {
             .body(body_text)
             .header((
                 "Content-Type",
-                format!("{}", mime::TEXT_PLAIN_UTF_8).as_ref(),
+                mime::TEXT_PLAIN_UTF_8.to_string(),
             ))
             .build();
 
@@ -339,7 +339,7 @@ impl EmailBuilder {
             .body(body_html)
             .header((
                 "Content-Type",
-                format!("{}", mime::TEXT_HTML_UTF_8).as_ref(),
+                mime::TEXT_HTML_UTF_8.to_string(),
             ))
             .build();
 
@@ -379,7 +379,7 @@ impl EmailBuilder {
         }
         // Add the sender header, if any.
         if let Some(ref v) = self.sender {
-            self.message = self.message.header(("Sender", v.to_string().as_ref()));
+            self.message = self.message.header(("Sender", v.to_string()));
         }
         // Calculate the envelope
         let envelope = match self.envelope {
@@ -471,7 +471,7 @@ impl EmailBuilder {
         if !self.date_issued {
             self.message = self
                 .message
-                .header(("Date", Tm::rfc822z(&now()).to_string().as_ref()));
+                .header(("Date", Tm::rfc822z(&now()).to_string()));
         }
 
         self.message = self.message.header(("MIME-Version", "1.0"));