After fresh installing Plesk 11 on Debian 6 I had problem with sending emails, so I comment out the line "smtpd_milters" in main.cf. In this case autoreply should working properly?
What exact problem do you have now? Autoresponder only or you cold not send all emails?
Try to switch MTA to Qmail and switch back to Postfix http://kb.parallels.com/en/5801
If it was not helpful provide more information from mail log:
/usr/local/psa/var/log/maillog
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