<form onsubmit="return validateEmail()">
  <label for="email">Email:</label>
  <input type="email" id="email" name="email" required>
  <span id="error" style="color:red;"></span>
  <br>
  <button type="submit">Отправить</button>
</form>

function validateEmail() {
  const emailField = document.getElementById("email");
  const error = document.getElementById("error");
  const emailPattern = /^[^ ]+@[^ ]+\.[a-z]{2,3}$/;
 
  if (!emailField.value.match(emailPattern)) {
    error.textContent = "Пожалуйста, введите корректный email.";
    return false;
  }
  error.textContent = "";
  return true;
}