WordPress multisite PHP mailer override for use with transactional email service

phpmailer.php

<?php

  //this file goes in /wp-content/mu-plugins
	
  //https://gist.github.com/butlerblog/c5c5eae5ace5bdaefb5d
  //constant configs in wp-config.php

  add_action( 'phpmailer_init', 'use_smtp_email' );

  function use_smtp_email( $phpmailer ) {
    $phpmailer->isSMTP();

    //https://deluxeblogtips.com/send-html-emails-in-wordpress/
    $phpmailer->IsHTML( true );

    $phpmailer->Host       = SMTP_HOST;
    $phpmailer->SMTPAuth   = SMTP_AUTH;
    $phpmailer->Port       = SMTP_PORT;
    $phpmailer->Username   = SMTP_USER;
    $phpmailer->Password   = SMTP_PASS;
    $phpmailer->SMTPSecure = SMTP_SECURE;
    $phpmailer->From       = SMTP_FROM;
    $phpmailer->FromName   = SMTP_NAME;
  }

wp-config.php

<?php

  //this gets added to wp-config.php

  define( "SMTP_USER",   "user-id-here" );
  define( "SMTP_PASS",   "user-pass-here" );
  define( "SMTP_HOST",   "smtp.postmarkapp.com" );
  define( "SMTP_FROM",   "[email protected]" );
  define( "SMTP_NAME",   "Network Name" );
  define( "SMTP_PORT",   "587" );
  define( "SMTP_SECURE", "tls" );
  define( "SMTP_AUTH",    true );
  define( "SMTP_DEBUG",   0 );