Plesk 12.0.18
Postfix+Courier-IMAP
Mail is sent from the Roundcube perfectly. When sending a response, it gives an error
554 5.7.1 : Relay access denied
Help solve the problem
If you still experience issues/errors/problems, try to include depending configuration files and don't forget to include MORE from your mail - logs ( serverwide AND from roundcube ) for further investigations.