Lock mutex before checking connection in Send method

Added mutex locking in the `Send` method for both `client_120.go` and `client_119.go`. This ensures thread-safe access to the connection checks and prevents potential race conditions.

client_119.go

@@ -11,6 +11,9 @@ import "errors"
 
 // Send sends out the mail message
 func (c *Client) Send(messages ...*Msg) error {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
 	if err := c.checkConn(); err != nil {
 		return &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
 	}

client_120.go

@@ -15,6 +15,7 @@ import (
 func (c *Client) Send(messages ...*Msg) (returnErr error) {
 	c.mutex.Lock()
 	defer c.mutex.Unlock()
+
 	if err := c.checkConn(); err != nil {
 		returnErr = &SendError{Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err)}
 		return