diff --git a/smtp/smtp.go b/smtp/smtp.go
index f9c5285..d2a0e64 100644
--- a/smtp/smtp.go
+++ b/smtp/smtp.go
@@ -210,7 +210,8 @@ func (c *Client) Auth(a Auth) error {
 	}
 	resp64 := make([]byte, encoding.EncodedLen(len(resp)))
 	encoding.Encode(resp64, resp)
-	code, msg64, err := c.cmd(0, strings.TrimSpace(fmt.Sprintf("AUTH %s %s", mech, resp64)))
+	code, msg64, err := c.cmd(0, "%s", strings.TrimSpace(fmt.Sprintf("AUTH %s %s", mech,
+		resp64)))
 	for err == nil {
 		var msg []byte
 		switch code {
@@ -238,7 +239,7 @@ func (c *Client) Auth(a Auth) error {
 		}
 		resp64 = make([]byte, encoding.EncodedLen(len(resp)))
 		encoding.Encode(resp64, resp)
-		code, msg64, err = c.cmd(0, string(resp64))
+		code, msg64, err = c.cmd(0, "%s", resp64)
 	}
 	return err
 }
diff --git a/smtp/smtp_test.go b/smtp/smtp_test.go
index 148ecf7..6df6eeb 100644
--- a/smtp/smtp_test.go
+++ b/smtp/smtp_test.go
@@ -1042,12 +1042,12 @@ func TestSendMail(t *testing.T) {
 
 		tc := textproto.NewConn(conn)
 		for i := 0; i < len(data) && data[i] != ""; i++ {
-			if err := tc.PrintfLine(data[i]); err != nil {
+			if err := tc.PrintfLine("%s", data[i]); err != nil {
 				t.Errorf("printing to textproto failed: %s", err)
 			}
 			for len(data[i]) >= 4 && data[i][3] == '-' {
 				i++
-				if err := tc.PrintfLine(data[i]); err != nil {
+				if err := tc.PrintfLine("%s", data[i]); err != nil {
 					t.Errorf("printing to textproto failed: %s", err)
 				}
 			}