function bp_activity_at_message_notification( $activity_id, $receiver_user_id ) {
	$notifications = BP_Core_Notification::get_all_for_user( $receiver_user_id, 'all' );

	// Don't leave multiple notifications for the same activity item.
	foreach( $notifications as $notification ) {
		if ( $activity_id == $notification->item_id ) {
			return;
		}
	}

	$activity     = new BP_Activity_Activity( $activity_id );
	$email_type   = 'activity-at-message';
	$group_name   = '';
	$message_link = bp_activity_get_permalink( $activity_id );
	$poster_name  = bp_core_get_user_displayname( $activity->user_id );

	remove_filter( 'bp_get_activity_content_body', 'convert_smilies' );
	remove_filter( 'bp_get_activity_content_body', 'wpautop' );
	remove_filter( 'bp_get_activity_content_body', 'bp_activity_truncate_entry', 5 );

	/** This filter is documented in bp-activity/bp-activity-template.php */
	$content = apply_filters_ref_array( 'bp_get_activity_content_body', array( $activity->content, &$activity ) );

	add_filter( 'bp_get_activity_content_body', 'convert_smilies' );
	add_filter( 'bp_get_activity_content_body', 'wpautop' );
	add_filter( 'bp_get_activity_content_body', 'bp_activity_truncate_entry', 5 );

	// Now email the user with the contents of the message (if they have enabled email notifications).
	if ( 'no' != bp_get_user_meta( $receiver_user_id, 'notification_activity_new_mention', true ) ) {
		if ( bp_is_active( 'groups' ) && bp_is_group() ) {
			$email_type = 'groups-at-message';
			$group_name = bp_get_current_group_name();
		}

		$unsubscribe_args = array(
			'user_id'           => $receiver_user_id,
			'notification_type' => $email_type,
		);

		$args = array(
			'tokens' => array(
				'activity'         => $activity,
				'usermessage'      => wp_strip_all_tags( $content ),
				'group.name'       => $group_name,
				'mentioned.url'    => $message_link,
				'poster.name'      => $poster_name,
				'receiver-user.id' => $receiver_user_id,
				'unsubscribe' 	   => esc_url( bp_email_get_unsubscribe_link( $unsubscribe_args ) ),
			),
		);

		bp_send_email( $email_type, $receiver_user_id, $args );
	}

	/**
	 * Fires after the sending of an @mention email notification.
	 *
	 * @since 1.5.0
	 * @since 2.5.0 $subject, $message, $content arguments unset and deprecated.
	 *
	 * @param BP_Activity_Activity $activity         Activity Item object.
	 * @param string               $deprecated       Removed in 2.5; now an empty string.
	 * @param string               $deprecated       Removed in 2.5; now an empty string.
	 * @param string               $deprecated       Removed in 2.5; now an empty string.
	 * @param int                  $receiver_user_id The ID of the user who is receiving the update.
	 */
	do_action( 'bp_activity_sent_mention_email', $activity, '', '', '', $receiver_user_id );
}