HTML in s2Member emails

I’m aware that this is a perennial question for s2-generated emails (like the welcome email) so I have spent quite some time today looking at what has been said in the community. The best response seems to be in a post form last year, in particular this contribution from leopol001 here: http://www.s2member.com/forums/topic/html-in-welcome-emails/#post-13320.

So what I have done is created /wp-content/mu-plugins/s2-hacks.php and then entered this code to set the content type used by wp_mail to html.

This seems to work nicely, but in the WP Codex (http://codex.wordpress.org/Function_Reference/wp_mail#Parameters), I read

Be careful to reset ‘wp_mail_content_type’ back to ‘text/plain’ after you send your message, though, because failing to do so could lead to unexpected problems with e-mails from WP or plugins/themes.

How do I reset the content type ?

Thanks,

R.