Account postmaster@localhost is missed in MailEnable Management.
Go to MailEnable Management > Messaging Manager > Post Offices and check whether postmaster user exists under localhost domain.If postmaster user does not exist:
Delete localhost domain from MailEnable Management.
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 MailEnable Management.
According to current business logic it is not possible to use both Plesk auto responder and option „Authenticated senders must use valid sender address“ enabled. As workaround enable the option and use autoresponder of Mail Enable.
Disable Authenticated senders must use valid sender anddress and Disalbe all catchalls in the Security settings of the SMTP Properties
Add the IP address of the mailserver to the Access controll exception.
Restart the SMTP Connector