diff --git a/smtp/smtp.go b/smtp/smtp.go
index 943ca9d..4841ec8 100644
--- a/smtp/smtp.go
+++ b/smtp/smtp.go
@@ -432,6 +432,8 @@ func (c *Client) Data() (io.WriteCloser, error) {
 	return datacloser, nil
 }
 
+var testHookStartTLS func(*tls.Config) // nil, except for tests
+
 // SendMail connects to the server at addr, switches to TLS if
 // possible, authenticates with the optional mechanism a if possible,
 // and then sends an email from address from, to addresses to, with
@@ -473,6 +475,9 @@ func SendMail(addr string, a Auth, from string, to []string, msg []byte) error {
 	}
 	if ok, _ := c.Extension("STARTTLS"); ok {
 		config := &tls.Config{ServerName: c.serverName}
+		if testHookStartTLS != nil {
+			testHookStartTLS(config)
+		}
 		if err = c.StartTLS(config); err != nil {
 			return err
 		}