Custom Contact Form giving Timeout error

I do like this plugin and on my computer everything seems to work fine. But I’ve gotten reports from my client that a couple people are getting timeout errors when they try to submit no matter how quickly they fill it out. I’ve tried it on Firefox and Safari and it’s fine. Would this be an Internet Explorer issue? How can I fix this? It’s a pretty big deal and I’m taking heat over this.

Can anyone help on this? THANKS!!