Hello Mike, first of all i would like to congrats you on this awesome script, it has made my life easier :) but there is a serious problem with your autoresponder.
Normally when you choose to send an HTML it should be sent also with a text only version of the message. Unfortunately your script does not have the ability to do both of them at the same time (when selecting html option).
In result you get from spam assassin quite a few negative points, this error: "SPAM: BODY: Message only has text/html MIME parts" and sends the email straight to spam folder and if you continue doing this you will eventually be blacklisted (your email ip in general).
The below link shares how you should send a multipart email (html&text version together).
Is this possible to be fixed by you?
P.S i am mostly using the PHP version of Fast Secure Contact Form Version: 3.1 but i assume its the same for the WP plugin.