Here's the email. Feel free to copy and paste it (and edit it as you see fit).

CLIENT NAME,

I will be out of the office from DATE LEAVING to DATE RETURNING but wanted to assure you that I have handed off all important information off to DELEGATE’S NAME while I am away. Should you have any questions or need anything during this time, please feel free to contact DELEGATE’S NAME and they will do their best to assist you.

Any correspondence (email or phone calls) sent my way will be responded to within 2–3 days of my return.

Sincerely,
YOUR NAME

Here's the blog post I wrote about this email.