http://support.microsoft.com/kb/311107/en-us

Using Outlook as a mail server results in unreliable operation
because:
- You must leave Outlook running.  If there is an error during a mail
poll, Outlook may pop up a dialog which will pause further mail polls
until closed.
- You need to leave Outlook always running.
- You need to leave Windows always loaded.
- You need to leave the computer always powered up.
- Your Internet connectivity must always be available.

Use the webmail interface to your e-mail account to see if it already
includes an auto-responder.  Server-side auto-responders are far more
reliable than client-side hacks.