Contact Form not working sometimes

Hi,
The form in my website sometimes is working and sometimes not. The error message is in orange border and stated ‘Failed to send your message. Please try later or contact the administrator by another method’. Most of the time are working fine, but sometimes will encounter this problem, any way to fix this? At the moment, I cleaned cache once a week. Thanks.

The page I need help with: [log in to see the link]