Unable to configure auto reply in Horde webmail.
According to Plesk 12 Release Notes configuration of auto reply in Horde is only possible if Dovecot is set as IMAP/POP3 server.
Switch mail server to use Dovecot as mail delivery agent:
Log in to Plesk.
Tools & Settings > Updates and Upgrades > Add/Remove Components > Mail hosting features
Select Dovecot secure IMAP server with Sieve support as IMAP/POP3 server.