diff --git a/Dockerfile b/Dockerfile
index a689d41..312afe8 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -44,10 +44,19 @@ RUN { \
     echo '        sender = *from'; \
     echo '    }'; \
     echo ''; \
+    echo '    debug := os.Getenv("DEBUG") != ""'; \
+    echo '    if debug {'; \
+    echo '        fmt.Fprintf(os.Stderr, "[sendmail] relay=%s port=%s sender=%s recipients=%v\n", relay, port, sender, recipients)'; \
+    echo '        fmt.Fprintf(os.Stderr, "[sendmail] body:\n%s\n", body)'; \
+    echo '    }'; \
+    echo ''; \
     echo '    if err = smtp.SendMail(relay+":"+port, nil, sender, recipients, body); err != nil {'; \
     echo '        fmt.Fprintln(os.Stderr, err)'; \
     echo '        os.Exit(1)'; \
     echo '    }'; \
+    echo '    if debug {'; \
+    echo '        fmt.Fprintln(os.Stderr, "[sendmail] sent successfully")'; \
+    echo '    }'; \
     echo '}'; \
     } > main.go
 RUN go mod init gomail && \