Astro,,
This can be done with the integration of a MTA that supports Quarantining. The Symantec DLP platform has the capability of modifying the Email Header (can be set to anything), this modification would then be recognized by the MTA for Quarantining. You would then get an email from either the DLP platform or the MTA that there is a message that is awaiting approval.
You would then be able to go to the MTA's login page and release the email to be sent out.
There is pretty seamless integration with Symantec's Messaging Gateway (see check box) and can do this with just about any other MTA that supports Quarantining.
Enclosed you will see how to configure the Response Rule that you would then associate to a policy.
It is pretty simple to do and is based off of Rules that you would have on your MTA.
Hope this makes sense.
If this solves your questions please marked as solved.
Ronak