User role email

When you create new user there is checkbox “Send the new user an email about their account.”. If you did not un-check it, WordPress itself send the notification email message. It’s built by this function:

/**
	 * Email login credentials to a newly-registered user.
	 *
	 * A new user registration notification is also sent to admin email.
	 *
	 * @since 2.0.0
	 * @since 4.3.0 The <code>$plaintext_pass</code> parameter was changed to <code>$notify</code>.
	 * @since 4.3.1 The <code>$plaintext_pass</code> parameter was deprecated. <code>$notify</code> added as a third parameter.
	 * @since 4.6.0 The <code>$notify</code> parameter accepts 'user' for sending notification only to the user created.
	 *
	 * @param int    $user_id    User ID.
	 * @param null   $deprecated Not used (argument deprecated).
	 * @param string $notify     Optional. Type of notification that should happen. Accepts 'admin' or an empty
	 *                           string (admin only), 'user', or 'both' (admin and user). Default empty.
	 */
	function wp_new_user_notification( $user_id, $deprecated = null, $notify = '' ) {

You can modify message content and other parameters via custom filter ‘wp_new_user_notification_email’:

$wp_new_user_notification_email = array(
			'to'      => $user->user_email,
			/* translators: Login details notification email subject. %s: Site title. */
			'subject' => __( '[%s] Login Details' ),
			'message' => $message,
			'headers' => '',
		);

		/**
		 * Filters the contents of the new user notification email sent to the new user.
		 *
		 * @since 4.9.0
		 *
		 * @param array   $wp_new_user_notification_email {
		 *     Used to build wp_mail().
		 *
		 *     @type string $to      The intended recipient - New user email address.
		 *     @type string $subject The subject of the email.
		 *     @type string $message The body of the email.
		 *     @type string $headers The headers of the email.
		 * }
		 * @param WP_User $user     User object for new user.
		 * @param string  $blogname The site title.
		 */
		$wp_new_user_notification_email = apply_filters( 'wp_new_user_notification_email', $wp_new_user_notification_email, $user, $blogname );

		wp_mail(
			$wp_new_user_notification_email['to'],
			wp_specialchars_decode( sprintf( $wp_new_user_notification_email['subject'], $blogname ) ),
			$wp_new_user_notification_email['message'],
			$wp_new_user_notification_email['headers']
		);