1. Click on the Mail tab 
2. Select an e-mail address
3. Click on the Auto-Reply tab 
4. Check the "Switch on auto-reply" check box and fill in the
"Auto-reply message text" and the "Forward to" e-mail
address and press the OK button.
screenshot
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How to enable Auto-Reply for an E-mail Address - Plesk 10
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Parallels Plesk Panel is a web hosting control panel that helps you manage your hosting account and assists you in creating web sites, managing e-mail addresses, & deploying applications to a server.
Plesk 10 is available on HostGator VPS, Windows Dedicated Servers, and Windows Shared Hosting.