Legitamate Emails from Form Going to SPAM

I am using the Product Inquiry form for my website http://www.jacksonarts.com

I have had customers complain that the email sent from the form has gone to SPAM and the email comes to me as SPAM. Can you tell me why this would happen and if there is a way to fix it?

Thanks – David

https://wordpress.org/plugins/product-enquiry-for-woocommerce/