Consult your email documentation.

For instance with Hotmail it is known as vacation replies and is off by default

• http://windows.microsoft.com/en-GB/hotmail/why-are-features-turned-off
• and may be turned on using
  options -> more -> Sending automated vacation replies

This answer is for HOTMAIL. I am requesting the procedure for Yahoo. Thanks

Sorry may telepathic powers are not too good ;-)

So look at your Yahoo instructions or Yahoo Answers

You can use the "Vacation Response" for that purpose.
On your mail page>>click on options>>mail options>>Vacation Response (from the menu list on the left).
On that page----check the box---"activate the auto response".
Fill in the form below with appropiate dates and message.
Click "save changes" top of page.

(From http://answers.yahoo.com/question/index?qid=20120124095048AAox6Lf )

Does that help ?

No I do not have the top menu that they are stating

Hey gd6784,

There are several versions of Yahoo Mail and they are each a bit different. Without knowing which version you use, it's difficult to help. I'd suggest you look at the Yahoo Mail support documentation as it will have more detailed information.

If you search for "out of office" in their documentation you'll see different articles that explain how it works and it will tell you which version of Yahoo Mail each article applies to.

Matt