Constantly gives "Failed to send message" error

Hi everyone. First WP Support post, please be genteel 🙂

I’m working on implementing the Contact Form on the website hopster.tv – and for some reason, while it was working fine earlier on, it’s suddenly spitting out the “Failed to send message” error. We’ve slightly changed the error messages, so it’s definitely not the spam error – but other than that, I don’t know how to debug it.

Tried disabling, re-enabling – unfortunately cannot uninstall/reinstall with the current permissions I’ve got (working on it).

Here’s the code I’m using
*FORM*

1. <label>Your Name:</label> [text* your-name]
2. <label>Email Address:</label>[email* your-email]
3. <label>Your Message:</label> [textarea your-message]
4. [submit “Send”]

*MAIL*
TO: [email protected]
FROM: [email protected]
SUBJECT: Message from domain.com
BODY
From: [your-name] <[your-email]>

Message Body:
[your-message]

—
This e-mail was sent from a contact form on Hopster.tv

***

Other plugins we’re using:
Google App Engine for WordPress
Google Analytics for WordPress
Facebook Comments
Display widgets
Advanced Custom Fields: Repeater Field
Advanced Custom Fields
Mailchimp for WP Pro
Soliloquy

Any thoughts on what could be going on here!?

http://ww.wp.xz.cn/plugins/contact-form-7/