As a network administrator I'm not a fan of the user quarantine. It means setting all users up with access to this feature, and providing training to the more technologically challenged users.
We have an Exchange server. When it scans an email for spam it adds a line to the internet header which reads "X-MS-Exchange-Organization-SCL: 0", with the 0 representing how confident the server is that the email is spam, and could be a number between 0 and 9 (9 being "this is definitely spam"). Additionally, within the spam detection settings there is an option to redirect any email with an SCL greater than a chosen number directly to the users' Junk folder. Could you add this line to the headers as the email passes through the scanner, so that any spam went to our users junk folder rather than your separate quarantine?
Failing that, could you simply prepend the subject line of any identified spam with some optional text, such as **SPAM** for example? We could then just set up an Outlook rule which diverts these emails to their junk folders.