Auto-Reply E-mails

Hi,

I have an online business from which I sell downloadable products, basically the client pays through a shopping cart system and my website sends an e-mail with the download links.

The problem is I'm the only one administrating the transactions and when I sleep at night there are clients that have issues with the e-mails not being received, many times they have powerful spam software that rejects the e-mail that my site sends out to them. I can only become aware of these issues when I wake up the next day to read the angry e-mails they send me, often they try to dispute the transaction online because no one is answering them and they think they got scammed or something.

I was wondering if there was a way to create auto-reply message to e-mails that contain specific keywords (i.e: This mail is sent via contact form on www.mysite.com) at specific times of the day. This way at night when I'm asleep and they try to contact me, a response will be sent explaining that someone will get back to them at a specified time.

Would this be possible somehow, is there such a service?