When trying to release any quarantined emails from the "quarantine" section at the web management, nothing happening and you are getting "Failed to release" alert.
The quarantine service uses its own SMTP configuration to send reports and send the released emails, he is not using the SMTP configuration releated to the Mail Agent.
To configure the SMTP configuration to the quarantine service that will give you the ability to release emails, you will need to open the web management console> Quarantine>Configure Quarantine Reports.
Under "SMTP Configuration Settings" section press "Update" and add your internal SMTP configuration and then press "SAVE".
Now try again to release an email from the quarantine.