Messages that show in green in the Quarantine+ tool have been accepted for delivery by your mail server. We store delivery logs for each message as provide proof of delivery. To track down a message, first click the "Troubleshoot" link below the message:

Then click "Was not received" on the next screen:

For messages that have been accepted by your mail server, a "Proof of delivery" will be displayed, which usually includes tracking information provided by your mail server upon accepting the message:

Inside the response, most mail servers provide tracking information. This will usually be a string of numbers and characters (sometimes including hyphens) and is provided by your mail server to make it easier to trace the delivery of the message.
 
The tracking information may look slightly different based on what type of mail server you have.

Copy the tracking information and send it to your mail server administrator and ask them to trace the message to discover where the message went after it was delivered to your mail server.