Smartermail autoresponder does not send an autoreply. Plesk and SmarterMail log files do not have entries about attempts to send an autoreply.
Account postmaster@localhost is missed in SmarterMail.
Log in to SmarterMail as administrator and check whether postmaster user exists under localhost domain. If postmaster user does not exist:
- Delete localhost domain from SmarterMail.
- Disable and enable option Use the mail server installed with Plesk and the default mail account created by Plesk in Plesk > Tools & Settings > External SMTP Server.
Domain postmaster@localhost will be recreated automatically in SmarterMail.
If it does not help check this article 125250.