add_action( 'messages_message_before_save', 'mycred_pro_intercept_bp_message' );
function mycred_pro_intercept_bp_message( $message ) {

	// We need recipients and myCRED
	if ( empty( $message->recipients ) || ! function_exists( 'mycred_get_users_rank' ) ) return;

	// The message sender
	$cui = bp_loggedin_user_id();

	// Get the senders Rank ID so we can get the ranks minimum balance requirement
	$senders_rank_id = mycred_get_users_rank( $cui, 'ID' );
	$senders_min = get_post_meta( $senders_rank_id, 'mycred_rank_min', true );

	// Now we loop through the recipients
	$good_recipients = array();
	foreach ( $message->recipients as $recipient ) {

		// Get the recipients rank ID so we can get the rank minimum balance requirement
		$recipients_rank_id = mycred_get_users_rank( $recipient->user_id, 'ID' );
		$recipients_min = get_post_meta( $recipients_rank_id, 'mycred_rank_min', true );

		// If recipient is on a higher rank, continue to the next
		if ( $recipients_min > $senders_min ) continue;

		// Add recipient to the "good" list
		$good_recipients[] = $recipient;

	}

	// Replace set recipients with the ones we have approved
	// If this results in an empty array (all recipients declined) 
	// BuddyPress will show an error to the user and no message is sent.
	$message->recipients = $good_recipients;

}