To have Yahoo! Mail reply to emails automatically while you're out of the office:
>> Select Options | Mail Options... in Yahoo! Mail.
>> Go to the Vacation Response category.
>> Make sure Enable auto-response during your vacation is checked.
>> Specify your auto-responder's start and end date under Auto-respond from and up until (and including) respectively.
>> Type the desired response you wish to be sent to all incoming mail under Message.
* It's good to include a note on when you expect to be back — and be able to respond personally.
>> Click Save Changes.
Set Up a Vacation Auto-Reply in Yahoo! Mail Classic
To set up a custom vacation auto-response in Yahoo! Mail Classic:
>> Select Options from the to Yahoo! Mail navigation bar.
>> Select Vacation Response (under Management).
>> Specify the start and end date of the auto-responder under Start Date: and End Date.
>> Enter the message to be sent in response to all incoming mail under Generic Response.
* You can also specify another message that will be sent in reply to mails from up to two special domains, for example that of your employer.
Unfortunately, Yahoo! Mail's auto-reply system does not remember who a vacation reply was already sent to, so repetitive mailers (but also mailing list admins) will get repetitive replies. Maybe they deserve it.