Why is just one of my first forms replies ending up in people's Spam folder?

This guide will help you to avoid your emails bouncing or going to the spam or junk folders: http://www.jotform.com/help/208-How-to-setup-email-alerts-to-prevent-email-bouncing-related-issues. 

If the issue still persists, you would need to ask your email provider to white-list our IP addresses, you will find them on this link: http://www.jotform.com/help/145-Whitelisting-JotMails-IP-Addresses.

Hope this helps.