Hello,
It looked like mails are already release. But in your case you said that clients couldn't get the mails released from Quarantine. You can change Administration/Control Center/SMTP/ and fill the "Define New Host" settings to send mails like notifications,quarantine release mails etc. from another SMPT like your Exchange server.
You need to give relay permissions in your SMTP also to accept connections from SMG.
Regards,
Oykun