I can't replicate this error. What email program are you using? What Zen Cart version? What version of the mod? Do you have any other mods installed which you think might cause a problem?
For some reason you have undefined constants which
email/email_template_autoresponder.html is trying to use.
Most of these constants should be defined in either:
includes/languages/english/email_extras.php
or
includes/languages/english/
your_template/email_extras.php
Can you check to see the constants are there?
What's strange is that $EMAIL_MESSAGE_HTML should be defined by the mod itself. This makes me think there's a wider problem, one which at the moment I'm not sure what the cause would be.
Perhaps you could try using another admin email address to test with? In autoresponder.php, find this code:
$admin_email=STORE_OWNER_EMAIL_ADDRESS;
Change to something like..
$admin_email="
[email protected]";
Related Tutorial/FAQ:
https://www.zen-cart.com/tutorials/index.php?article=95