to me the warning message says what you are after: "Enabling the digest message will NOT result in emails being sent OUTSIDE your organisation" (i.e. to users not part of your email domain).
Or am I missing something?
2: Is it a feature to enable quarantining ONLY to users in your email domain?
I'm assuming you are talking about McAfee Email Gateway (MEG) version 7.x.
I'm pretty sure that by default any message in quarantine (regardless of the receiptient) will get a notification. So you want to make sure you setup the notification to only get delivered to the internal server or domain. From within the Quarantine Digest Options screen, check your Protocol preset. You probably want to make sure Destination IP address is in range (for whatever your internal network is).
You should double check with support, but I think that works.
In this case I am talking about both products. I have a MEG7 installation and EWS5.6 - the EWS installation being the more established one having been in place for about 18 months.
In the end I have used a protocol preset referencing the IP address of my Exchange server - and it appears to work as intended. I was trying to establish whether doing it this way was correct or whether there was a different or better way of configuring this.