Disables email from sending on a WPMU Sitewide Tags blog.

disable-email-on-sitewide-tags.php

<?php
/*
Plugin Name: WPMU Sitewide Tags - Disable Email
Description: Disables email from sending on a WPMU Sitewide Tags blog.
Author: r-a-y
Version: 0.1
*/

/**
 * Disables emails from firing on the WPMU Sitewide Tags blog.
 *
 * @param int $new_blog The new blog ID that is being switched to.
 * @param int $prev_blog_id The current blog ID before switching.
 */
function ray_disable_email_on_sitewide_tags_blog( $new_blog, $prev_blog_id ) {
	if ( ! function_exists( 'get_sitewide_tags_option' ) ) {
		return;
	}

	$sitewide_tags_blog_id = get_sitewide_tags_option( 'tags_blog_id' );

	if ( $new_blog == $sitewide_tags_blog_id ) {
		add_action( 'phpmailer_init', 'ray_disable_email_content', 999 );
	} else {
		remove_action( 'phpmailer_init', 'ray_disable_email_content', 999 );
	}
}
add_action( 'switch_blog', 'ray_disable_email_on_sitewide_tags_blog', 10, 2 );

/**
 * Wipes out the email content when sending an email.
 *
 * This will disable email sendouts since the email body will be empty.
 *
 * @param object $phpmailer The current $phpmailer object.
 */
function ray_disable_email_content( $phpmailer ) {
	$phpmailer->IsHTML( false );
	$phpmailer->AltBody = '';
	$phpmailer->Body    = '';
}